avast doesn't have a fixed VPS update schedule, but the program checks for updates every 4 hours (unless you have changed the settings) and if one is available you can download it or let the auto updates do it for you.
On average there are about 3-4 updates per week and as you mention some times there are many in a single day, I believe I have had 4 updates on one day. This entirely depends on activity and seriousness of any sample to be included in the VPS.
So far there have been 3 updates in this weekly cycle.
