There are no set frequency for updates, they are released as required and ready. See avast! VPS Updates History to get an idea of the frequency.

The reason for asking what VPS update you had was to try and find out how long (if at all) the VPS updates haven’t been working and try to get to the bottom of why they aren’t working, that no longer is an option.

Depending on your connection method, avast checks for updates each time you connect to the internet and again after 4 hours if you remain connected, if one is available it will be downloaded and installed.