You should back up often enough so that you can recover data that's still current or can be made current with minimal work.
In a software development group, this may range from a day to a week. Each day of out-of-date backup will generally cost you a day of redevelopment. If you're saving financial or point-of-sale data, then daily or even twice-daily backups are common. It's a good idea to maintain off-site storage.