In general you should keep a daily backup for each day of business, a weekly backup, and monthly backups for a minimum of 6 months. This is of course dependent on how many transactions you perform each day and how much administrative and management work (word processing, bookkeeping, documentation, etc) has changed in your systems each day.   But the rule still generally applies. It also depends on how much it would cost you to redo the work if it was lost and had to be reentered.

For Example:
You would have individual backups for:
Monday is your weekly (you would have 4 different Monday backups - one for each week of the month)
Tuesday daily
Wednesday daily
Thursday daily
Friday daily
Monthly backup (you would have a minimum of 6 monthly backups) and would use this backup instead of any other backup on the last business day of the month.

Companies that have huge transaction volumes will usually backup more often and use different methods.

Offsite storage is any place off the premises of your business.  When we say off the premises we mean out of the same physical building that you're business is located in.  Actually it should not even be in the same city block as your business.
This is one of many reasons that organizations are turning to using the cloud for the backup storage.
WHY - the two most common reasons - fire and theft.  If the building burns down, or the computers stolen or vandalized, where's your A/R?  (A/P can wait a little bit but not too long. ;)

Some other considerations - do you know what to do with your backup tape that you brought home with you if your office did suffer a disaster such as a fire.  Remember your plan better have a way of retrieving your backup data!