It seems to kind of vary, actually. Sometimes it's a half hour( seems in the morning and mid-afternoon it updates more quickly) but other times I've seen it be 2-4 hours without any update, or with only updating impressions and not clicks. Often times it seems to be on the 1-2 hour cycle, particularly later on in the evening, and sometimes later on in the morning as well. Other days, well, it's just a who-knows
A general rule of thumb I follow - If you login to adsense, then refresh the page - if it asks you to login again your stats have been updated. If you're still logged in with your current session chances are they haven't updated at all.. It's always worked for me, but I might not check quite as often as you might..
i just go to advanced reports and the #'s there is always more updated than what's shown when u first log in.
I think it varies from account to account. You will find some people complaining why their adsense stats haven't updated on a given day and others saying they are not seeing any problems. I always felt that Saturdays are somewhat slower than other days though.