I backup my promotion forum once a day. With my bigger forums I do 3 backups a day. All my backups are done automatically via cron. I then test the backups on a test server. I usually keep 2 weeks worth of backups.
I recommend for all backups to be done via SSH/Telnet. If you have a large forum don't use the back up feature on your forum.
I was wondering how when you guys backup your forum database and when you backup your forum files? Not just whether you do it periodically but when do you backup as a precaution? Before administrating forums or users? When installing mods?