system
19
You are confusing Avast checking for availability, and actually finding an update to apply.
Using the default settings, Avast Free will check every 4 hours after booting, and only part of those attempts it actually will find a new database to apply.
In addition, 30 minutes is not allowed, it’s too frequent for the Free edition.
You want to check in Avast → Summary. If the database version is in green, it is the latest available, even when the latest attempt was 30 minutes ago.