It checks for updates when you connect to internet and than every 4 hours. This is optimal interval. For each "blank" check (eg when no new update available) it uses only 3Kb of bandwidth so its practically nothing.
You can also use manual VPS update via trey icon anytime you want.
If you have avast! Pro,than you can set update scheduling task for VPS update on desirable time of the day or of the week