How often should we test a restore?
At least once a quarter for a sample of systems, after every significant change to the backup setup, and at least once a year as a full exercise of restoring a critical system from scratch. A backup that has never been restored is an assumption, not a backup: jobs that report success can still produce files that are incomplete, encrypted with a lost key or missing a database that was in use. A good test picks real systems, restores them to an isolated environment, checks that the application actually starts and the data is current, and records how long it took. That duration is your realistic recovery time, which is often much longer than expected. Write down what went wrong and fix it before the next test. Rotate which systems you test so that, over a year, every critical one has been restored at least once.