It all depends on the data... how much risk are you willing to take on it?
I backup my Mac automatically to a Time Capsule. That way I always have a backup that is 1 hour out of sync at most. Then about every 3 to 4 weeks I sync my Photo's, Music, Documents and Development folders to an external 2.5" USB harddrive that I have put somewhere safe. And about once or twice a year I burn all Photo's to DVD's and store them with my parents...
