(By changing I mean choosing a new outfit for a certain category, not changing into pyjamas or something.)
I normally don’t do it often. I find a perfect outfit for a certain sim and they keep it for their whole life (i.e. forever considering that my sims don’t die). Moreover, if I happen to have the same clothes for multiple ages (thanks Skell and other people who make age conversions!), I’m able to find a perfect outfit for them in their teen or even child years, and I choose the same one when they age up. What about you, do you stick to a single outfit for as long as you can, or change your sims’ looks more often?

When the season changes I like to give them something appropriate. On aging and in-between aging since my sims have extra long life stages. I have a metric ton load of clothes...

On average my sims get a minimum of two outfits per season and some fashionista types get even more. I have a metric ef-ton of cc clothing so no one really gets the same outfits either (and I have over 100 playable characters). So I guess to answer your question: "A lot"

They're supposed to change clothes?

My sims generally change clothes immediately after aging up. Okay, but really...I tend to pick one outfit that I feel suits them and then they just stay in that. I wish they'd actually roll wants to "buy new clothes" more often, because then I'd feel like it wasn't a waste to actually do that.

That's a character thing. Someone like Bad News Bev may buy far more clothes than she needs and change every day; someone like Red Onions may wear one outfit per lifestage. Budding fashionista Dior Skirt changes clothes once or twice per day and now that he can sew clothes he'll be trying to make outfits rather than buying them. Most people change their clothes for a changing season, but not all my hoods have four seasons. In a university that's only got spring/fall/spring/fall, one student may keep the same outfit for every semester because the temperature is pretty even and why else would you change? While another may insist on wearing spring colors in spring and fall colors in fall. A lot of college students are experimenting, and changes of clothing or hairstyle may mark changes in maturity or self-image. It's fairly common to get a complete makeover on joining a Greek, getting engaged, moving from a dorm to a residence, changing aspiration at Junior year, etc.

Teens and YAs probably change clothes more often than all other aspirations combined, as they're the age groups that are busiest experimenting and developing their identity. Elders probably change clothes less than anyone but toddlers, simply because the choices for elders are so few, so most of them find a look and stick with it. Toddlers change least often because, even with all the cute toddler stuff out there, who has time to plan a new outfit for them every day?

Maybe every sim week or two. I have a lot of CC clothing that needs to actually see the light of day. XD

I have a lot of cc so mine change clothes everyday. The women have different evening gowns to wear for different things. The guys pretty much just change every day and depends on his wife or girlfriend he may own a tux or suit that matches her evening gown or may not.

Usually they just keep the clothes the game gives them when they age up, but if I really don't like those clothes, or if they want to buy new ones, or if their clothes look particularly inappropriate for the season, then I have them change. Also, I tend to furnish houses in colours that match the Sim's everyday clothing, but if they then end up with pj's that clash with their bed covers (e.g. orange pj's with a red blanket), they get new pj's.

Hardly ever. I wish Sims 2 had a function like 4 were you can designate a number of outfits and the Sim will randomly cycle through it.

Otherwise it tends to happen whenever 1) a sim gets that "buy new clothes" want 2) I can't stand looking at a Sim's look anymore and need to give them a makeover 3) Story reasons (their circumstances change and their look changes with them)

Never. In CAS, once I pick out whatever clothing I like best on my custom Sims, it stays permanent. I don't mind fulfilling their desire to purchase new clothes if it ever comes around to it (I won't have them change into them though), and very rarely I'll have them try out clothes for fun. Although it is not the same experience, it does somewhat reflect on the fact I used to despise clothes shopping when I was much younger.

Not very often - on ageing up, certainly, because the clothes are usually horrible or inappropriate. Sometimes I look at the season and change accordingly. When I'm trying to differentiate between a young child and an older one nearing teenhood I might change the clothes - the same with elders in their 50s compared to 70s.
If the Sim has lots of clothes in the wardrobe he/she will change more often, but mostly they only have one or two outfits and I can't be bothered to shop for more. If I've downloaded something particularly nice I'll make them go shopping. The males usually make do.
Edit: oh, and if the same formal wear keeps popping up at weddings I'll make them change.

But this way it's like someone made you go clothes shopping and then wouldn't let you wear what you bought.

I never change my sim's clothes while playing them. I can see how it would be fun though. The only time they change clothes is when they age up. Even then, I only ever change the clothes for teens and older. I might switch it up once in a while and give the kiddies some new clothes, but toddlers I really can't be bothered with, mainly because I neglect that life stage out of laziness. I'm so mean to my toddlers, and I really should give them more attention :p

Behold, a reason to give them more attention. XD https://www.modthesims.info/member.php?u=285828 I couldn't even visit the page without seeing something I had missed and had to have. XD

Sim Sample got into PVC some time around 2004. But by 2009 he was back in that old black T-shirt.

Those are so great. They're like the perfect clothes that I like my sims to wear. 305 different sets of clothing for the kids. I guess I have an excuse to change the kiddies clothes

My story sims often change clothes from chapter to chapter, and even several times in a single chapter. I like to show time passing, and changing clothes can help in showing it's not the same day. I also use clothes and hairstyles to a certain degree in showing age, so younger babies, toddlers and children will have "cuter" clothes, and will have less "cute" clothes as they grow, which means there's a difference between a newborn baby and a baby several months old, a toddler the age of 1 and 3-4, and a child the age of 5 and up. Clothes also help with showing seasons, and setting a particular scene.

Non-story sims tend to stick with their original clothes most of the time.

Like you, I rarely change them.
I love what I choose for my sims too much, to care changing them.
From teen to adult, sometime even elders(pajamas) they will wear the same thing, because I made it, and for all 3 ages.
I will also create color options..so some might wear the same dress, but in different colors. Or style. For instance, Camille wears my edited FT teen dress, re-meshed for adults...while her teen daughter Flo, wears another edit of the same dress..by me. In this case, I didn't even bother choosing another color. lol

I'm seconding JoandSarah,

I like to give my sims appropriate clothes for the season (and sometimes accessories as well). I usually use the FFS Clothing Tool to adjust the outfits that they spawn randomly when growing up etc. , and then from there I make them buy all the new ones. Everyone gets one freebie!

It's kind of fun though, because at the end of summer, my families go school shopping and the kidlets all get new, seasonally and school appropriate outfits. I also like to humour the want to 'buy new clothes', so whenever they get it, they get sent off to the shops to get something for themselves, and depending on personality, for the rest of the family too.

I pick their outfits based on personality, and income bracket and I've recently started downloading separates that are binned in more than one category so that they can really customize outfits (and you can buy jackets and such without buying bottoms as well). I've also been putting wardrobes and such in more of my houses as I want to have them change clothes every sim-week (so about every 5-7 months in their time).

Ummm - seasonally probably. I do change when it's sunny and they're still wearing a woolly jumper and scarf. They get new clothes on age up too - I allow them a free set then to circumvent EAxis's lovely choices.

Something different every day and sometimes several times a day depending on what's happening. Unless they have no money, and in my game that's extremely rare, they have many choices. Someone isn't going to wear the same outfit to confront her father that she is to go shopping with her best friend or to meet her boyfriend for lunch or to apply for a job or to attend a funeral. I have only 35 families after a lot of human years and have had about 15 children in all that time so it's no big deal for me to shop with them. I like clothes and enjoy it - sometimes more than they do.

I'm with Jain. unless they are dirt poor, or have aged up with only one outfit, my sims change clothes at least once a day, some two or three depending upon how often they have to shower.

Saturday is gussy-up day in all my hoods. My sims "go to town". Not really, I use the gussy-up mod and buy them free new clothes and treat them to new hairstyles. Because seasons are five days long, I pretend they are shopping to stock up for the next season.

I also dress them for the season. But the catch is, it's MY season not theirs! If I'm bundled up, sitting under my quilt playing in the dead of winter, I would freeze watching them in their sundresses and shorts. Even if it is high summer in their dessert, they are at least wearing sweaters and long sleeves!