How often should I test my backups?

Often enough that a failure surfaces while it is still cheap. For most small businesses that means:

• Once a quarter - restore one real file from a real backup and open it. Fifteen minutes.
• Once a year - a fuller rehearsal: restore enough to check the data is complete and usable, not just present.
• After any change - new device, new provider, new folder structure. Changes are what silently break backup coverage.

A backup job that reports success proves the job ran, not that the data can come back. Only a restore proves that.