You can edit the period in avast.ini - [VRDB] section, RunInterval value (if it's not present, just add "RunInterval=your_value" to the [VRBD] section). The value is in minutes, i.e. if you want one week, write RunInterval=10080.
As for the maximum protection - it's not clear whether shortening the interval as much as possible gives you more protection. What is necessary is to have uninfected files in the database, i.e. the state of the files before the infection has happened. If you refresh the database too often, some infected files might make it to the database before you notice you were infected (e.g. before the VPS gets updated).
Since VRDB holds multiple versions of the files, the risk is not big (except for the files that are often updated, but it is usually not the case with executables)... but in my opinion, running VRDB generation every hour is not a good idea either
