Quarterly or twice-a-year testing is a solid baseline for most organizations. If you have frequent system changes, you may need a tighter cadence.
Also test after major events, such as a server migration, a new EHR or accounting platform, a merger, a new office, or a security incident. The goal is simple: if the environment changed, the recovery steps should be revalidated.
Rotate scenarios. One cycle might focus on identity and file services. The next might focus on a core application stack. Over time, you build coverage across the systems that keep the business running.
Test both the technical restore and the operational workflow. Who declares an incident. Who communicates to staff and clients. Who approves a failover. Who confirms the business can transact again.