A health check is generally not done very often. A server health check is looking at the server configuration, settings, etc., to determine whether or not they are appropriate for the hardware. Additionally a health check can include some performance aspects such as looking at wait statistics on the server to understand where bottlenecks exist. The reason for this is because those bottlenecks can also suggest where to check that settings are correct.
As to frequency, I'd say any time you're new to a system, do a full examination of it, and then, any time hardware or settings are changed. Other than that, they're generally not needed. This assumes of course, the servers are under your control and you have good change tracking, source control, etc. If they're not under your control, maybe, monthly spot checks to see if anything has changed.