How often do I have to back up? And what we do we do with backups?

There is no rule. Whenever you want. There is also a function that let InterpretBank perform backups at regular intervals (menu Options/Preferences, tab Private Database, option "Automatic backup database at shut-down".

You should store your backup in a secure location, for example on an external hard drive. This is useful in case you lose your computer, get a virus, or accidentally crash the machine.