Minute by minute lately... or so it seems, depending on what datacenter is serving up the results at the moment. Everflux is constant. I seem to see a pattern of two minor to medium updates a day. Around 11am eastern US time, and sometime early morning (between 2am to 6am?). Then of course, major updates sometimes every few months or yearly. They seem to like doing a 'big' one right before the christmas shopping season begins. That one's a killer.
Get a good chunk of your keywords onto the Digital Point Keyword Tracker. Watch them for a week or so, and you'll see that your results will often change several times a day. It's like the stock market. You'll drive yourself nuts if you micro-watch your results.