I would always guess that adsense stats that give your earnings updated every 15-20 minutes and occasionally seem to get “stuck” for an hour or two. I used to check them tirelessly when I was getting started so I thought I had a pretty good grasp on when they updated.

Google officially tells us now how often they update (source):

many publishers have asked us why they don’t see any changes in their stats each time they log in, or why they sometimes see a ‘No data available’ message in their reports. This is because your reports are generally updated every 15 to 30 minutes, but there can sometimes be a delay of up to 24 hours. In addition, it can take up to 24 hours to fully verify our internal logs and finalize your reports — as a result, you may sometimes see changes in your earnings during the course of the day. Please also keep in mind that your reports won’t show data for impressions on public service announcements (PSAs).

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