If you're using amavis-ng, then AFAIK it isn't actually amavis that needs updating, it is whatever virus-scanner that amavis is using that needs updating. Amavis-ng is a kind of "middleman" between your MTA and your virus scanner.
For example, I'm using amavis with clamav. The clamav virus definition gets updated every hour.
If you're interested, this was all set up using Jesper's script (search this site, can't remember the URL).