CVR testing is included in the Approved Maintenance Programme, the interval at which it is performed is, or ought to be, governed by the operator's reliability monitoring programme. During the ten years or so that I worked in Base Maintenance, I found no more than three unserviceable units when performing these tests. During my subsequent twelve years in Technical Services we never found cause to either increase or reduce the testing interval.
Be aware that the CAM frequency response isn't ideal for voice and this microphone is not intended to pick up speech; it is primarily intended to record cockpit background sounds such as trim wheel operation, landing gear being lowered etc. A common problem for accident investigators is that, for obvious reasons of comfort, pilots often prefer to use the overhead speakers and hand microphones for communication. In this case, no continuous direct speech is recorded from the "Hot" mic on the headset, only when the transmit button is pressed.
On one airmiss incident that I was involved with, the crew had pulled the CVR circuit breaker to stop the recording of ATC transmissions being erased by overwriting - CVRs generally record only the last thirty minutes. During the subsequent transit stop somebody reset the circuit breaker and the evidence was lost - though we had thirty minutes of interesting conversation covering the let-down into home Base.