-Is it possible to skip any levels based on prior experience?
These days, probably not. People are going from the Second City mainstage or IO mainstage Harold Teams to 101 at UCB. Going back to basics isn't going to hurt you, though.
-How often do you get to perform (in class and on stage )?
Your first three classes will have one student show, and the 401 class has two. In class, the first two levels are all scenework and exercises. Class sizes run about 15 or 16 people and the classes are 3 hours, so you won't have any weeks where you don't get up at all. You'll be doing probably one Harold per class in 301, along with scenework in the first half of class. I can't speak for 401 yet.
Outside of class, there are also practice groups, encouraged from 201 on, TNT and Crash Bar, which you can submit a group for and try to get a slot in, UCB's Jam, and other non-UCB performance opportunities, like the Social, FNF and whatever the Saturday night show is at IO West, which I'd recommend. There's also, I believe, a weekly Jam after TNT.
-When can you audition for a weekly show?
After you finish the 401 class, you're eligible to audition for a Harold team. They just had auditions, though, so it'll be a while until they have them again. Once, maybe twice a year is usual.
------How likely is it that you get put in a weekly show?
That depends on you. There are a lot of students/grads, and usually only 5 or 6 Harold Teams.
------What weekly shows can you join at the above stated time? (ie Harold, or others)
The Harold Night shows are the only weekly shows you can audition for.
-Can you explain more detail of what each level teaches (besides what the website says)?
101 - Intro to improv - Yes, And; Environment work; basics of game and UCB style improv. Scenes from a suggestion and scenes from an opening which, in level one, is monologues. Student show is Suggestion, monologue, three scenes from that monologue, new monologue, three scenes from that monologue, etc.
201 - Finding the Game - More detailed examination of the game of the scene, with a focus on finding it quickly and heightening it. Intro to pattern game opening, first and second beats. Student show is suggestion, pattern game opening, three first beats, three second beats, followed by montage for the rest of time.
301 - Harold - Group games, third beats, putting together the full harold. New openings including the Invocation and the training wheels organic opening. Student show is a full Harold.
401 - Harold Workshop - more focus on the Harold. Someone who's taken this can give you a more detailed breakdown.
-Do you only learn Harold or do you do any other long forms?
See above.