How long does it take to graduate?

It depends on how many courses you take per term. One term lasts for a week and the maximum amount of courses you can do is four, netting you four credits (1 course = 1 credit) and you need twelve credits to graduate.

So, if you took the full workload each term you'd finish university in the shortest amount of time; three sim weeks. (4x3=12)

I would recommend turning aging off for sure. Also, some may find four courses per term very overwhelming. I did it on my first run through uni, but loosened up to 2-3 on my next sim. Four is doable and a fun challenge though.

I just wanted to add one little thing, unlike past Sims games, you can not create a sim and put them right into university, they will need to apply which will take a few sim days.

I would recommend setting aging to Long or turning it off. Completing just one degree will take most or all of your sim's young adult lifestage if aging is set to Normal or Short.

For the first degree that a sim goes for they have to have 12 courses completed. Each course involves 5 days (but they don't have classes every day for each class. So a sim may only attend in that 5 days two or three days before that course is completed. There are no classes on Saturday or Sunday as far as I have seen in my game.

It is up to the player on how many classes they want their sim to complete for the course period, meaning they can take 1, 2, 3, or 4 classes at one time, but I don't recommend trying to have your sim complete 4 classes for the first time a player sends sims to university. I did and found my game to be a bit overwhelming and not enjoyable to play. Also, keep in mind when picking the number of classes your sim still has other class assignments to do as well.

If the same sim who has completed their degree decides they want to go for another degree, any degree after will only require 8 classes.

There is something called a Distinguished Degrees if your sim completes those, and has top grades they can easily start out at a level 7 or 8 in the given career attached to that degree.

Carl Sims goes into even more detail on all of this. https://www.carls-sims-4-guide.com/expansionpacks/discoveruniversity/

A lot of time. If you go with maximum difficulty, 4 classes per semester, about 3 semesters, or 3 weeks.

Now I just have to come up with a Sim...New expansion pack + New computer = wanting to start over completely.
But as what, who, where? (Starting with a modded version of lilsimsies latest save, btw.