It can be updated on a daily basis, it depends on the number of computers they have to add. Sometimes the same item will appear multiple times in the same day. I followed it very closely a while ago and that's what I noticed, anyway.
I agree with mcpryon2. When I was following it closely recently, it changed very frequently. I'd say sometimes daily, sometimes more than once per day, but not every day.
When I was looking for a Mac mini sometime last year, they'd literally just randomly pop up and then disappear 10 minutes later. I'm not sure if they have a schedule, but I might just not be watching as closely as others on the forum.