I tend to play uni on its own rotation separate to what's happening in the main hood (university is a bit like its own world that way lol). With my ages, and some imagination, I view the YA stage and the Adult stage as being effectively the same. My YA sims are 18-22, and the first four years of my Adult stage is 18-22. The fact that sims who don't go to uni will live 'shorter' lifespans is something I imagine as a hypothetical in terms of 'experience' (lol).

So a day 1 uni fresher is the same as a day 1 adult in the main hood. Both 18.

I also don't play by strict rotations. I like to play out events to their fullest and then jump to wherever I need to to continue the action, so I keep rotations extremely loose. The only structured thing is where I start - I play clockwise from the 'first' house at the top of the screen, and continue from wherever I end the round until I've done a full 'circle'. (It forces me to play ALL of my families, and keeps any that are going through a 'lull' in the action from stagnating for ages while I play elsewhere.)

I play one-day rotations in the neighbourhood and one-semester rotations at university, so the equivalent would be for you to play students right through from freshers to graduation in one sitting if you want to keep the dates exactly straight. I wouldn't recommend it, though. University can get pretty dull, and playing eight semesters straight through would be deathly, especially if you had to do it with several student households.

I wouldn't worry too much about keeping the ages exact. People who leave school early and get on with their careers are going to be further along in "adult" life than those who take time out to study. They also spend more time in the adult stage, so you could use something like the Elixir of Life (if you want to keep it vanilla) to give a few extra days to those who go straight from high school to work. If you add those extra days at the beginning of the adult life span, you'll keep everyone pretty much in synch. Sims who don't attend university at all could take three swigs of Elixir, and those who drop out could take one or two, depending on how much time they spent as students.

I use Cyjon's shorter semesters mod, and I play one day rotations. In my game sims are YA's from the age of 20-29 (4 2-day semesters and one day at the end to throw a graduation party or whatever before becoming an adult) and, if a sim doesn't go to university, they spend that time as an adult. When a sim graduates from college and becomes an adult I use the SimBlender to deduct 9 days from their adult lifespan. If a sim drops out or is expelled before they finish their studies I deduct the number of days that they spent studying. This means that everyone's ages remain in sync.

I also play college within the main 'hood, so sometimes YA's live with their families or with other non-YA's whilst they are studying, so I need to be able to play mixed-age households without the ages going weird.

Because I don't send everyone to Uni and I want them to stay roughly the same age I use this system. Teen are teens for 8 days. On day 9 they either get to become an 'independent teen' (I have a mod that lets them stop school, get a job, and move out if they want) or go to Uni. I play one Uni semester for every day back home. I play each lot for one day then move to the next lot. However, I have very few independent teens. They either failed out of school so bad they couldn't get to Uni (D or F grades), or occasionally as teen pregnancy is VERY rare in my game they stayed home to raise a child, or have a want that has no need for Uni, like run a business. In the business case I'd rather have them start their first business so they can earn badges and build up the business rather than 'waste' time at Uni. As I mentioned before though it is very rare for a sim not to go to college. Happens to maybe 1 in 10.

As to your original question, one Uni semester for each Sims day.

Another thing I figured out - fortunately this time in the bathtub: If each semester is one Sim day, that would be eight days. Since teenagers are fifteen days - the ones who go to college go on their second week of teenhood, and the ones who remain teens just stay teens and possibly 'graduate' via Simlogical Flexi School (had trouble with SimBlender's 'independent teen', unfortunately).

I have UNi length set to be 8 days but I also add two more days from the 72 hours the game gives them after graduating. Likewise my teens also get 10 days. For me each sim day is 6 months so 10 days is 5 years. 13-18 is five years as teen and 18-23 is the 5 years as a YA.

Experimenting is probably the best way - I cannot imagine playing shorter Uni terms because I really, really love Uni. So what works for one may simply not work for another player.

University semesters spread over four years so I play the equivalent of two years (or four semesters) in one rotation, which equals the two days I play in the main neighbourhood.

When I really hit my stride in playing, I played one week (Sunday to Sunday) in the hood and one full year at University (6 days, I believe?). By that point, I had BoilingOil's Slow Aging Controller edited so that the different age stages had their own default aging days. While this kept sim ages from getting really out of whack, it did mean the seasons were out of sync. Since I've been playing students in the main hood, I've switched to season based. Rotations go from 1st day of the season to 1st day of the next season. It isn't really as long as I want to spend on each household, especially now that I'm using Hat's proportional aging mod. But since a household of students doesn't show the current day, it's just easier to remember to hop to the next on the 1st than try to remember the shifting day of the season I stopped on (2nd, 3rd, 4th??).