You can manually update the signatures from a local file (on disk or CD-R). If you login to support.fortinet.com with your account, there is a menu item " signature files" .
Depending on your choice the AV database can be up to 100 MB or more. Have a look at the Release Notes for FortiOS 4.x where these options are explained.
Once the AV and IPS databases are current the daily updates are incremental and very fast to download. But if I forget to update e.g. on my notebook which I don' t use daily then it might happen that after 3 or 4 days (no time span specified) I' ll get a full update. Which freezes my notebook for 15 minutes or so.
Ede
"Kernel panic: Aiee, killing interrupt handler!"