How often does Adsense update itself?

Answer: 15-30 minutes. I found this out by reading the Google Adsense b*log:

AdSense reports are usually updated every 15-30 minutes, but sometimes there can be a longer delay before your reports are fully updated. Even if you notice a delay in your account reporting, fear not — your stats are still being tracked and you’ll be paid for all valid clicks and impressions.