Well, yes the time can be changed by editing the configuration file, but why would you want to do so?
One of the finest features of Avast is the ability to keep your system protected, and it does so by updating the newest DB as soon as possible.
I would have thought you would want to set it lower than higher!
It would be more important for you to reset the VRDB from the 3 weeks setting. This way if your system does get infected, your chances of restoration are better since the backup of important system files is more current.
Most of us in the forum can direct you to a previous string on resetting the VRDB.
Good luck.