The program updates are released but to avoid excessive load on the servers the auto update check happens randomically every 12 hours and it's not checking every time you connect to the Internet as with the virus definitions checks. The cycle is controlled by the servers and can be changed at any time. So no later than 12 hours after the update is released, everyone should get at least one popup. The order is machine-fixed (not random-generated for each update).
Avast Free AV checks for updates every 4 hours. Avast Pro AV and IS check every 1 hour. The program already knows that the update is there, but it won't tell you until it's your turn. You have to allow the update, i.e., it's not a forced and automatic update.
They also use 3rd party servers for updates distribution, combined with their huge network of standalone servers, to distribute the content from geographically close locations.