Think of how much work you want to redo if a problem occurs and that might help you decide how often to back up. If you are working steadily for hours, you might want to do a backup after each half hour or hour or lunchtime and at the end of the day for example.

I recommend using Sage's backup - it is the easiest to restore if done right.

How long to keep backups: Depends on what your company requires. You probably don't have to keep every one of your older backups - maybe just quarterly ones for example. This needs to be discussed with management or accountants or auditors - depends on your company.