You will receive a variety of answers to your question because:
1. there are many different applications of relief valves; such as:
a. water heaters
c. pressure vessels for chemicals
d. refrigeration machinery
2. Each manufacturer of relief valves specifies on an attached tag the recommended frequency of opening.
3. A manufacturer of a machine may specify the frequency of examination aside from the valve manufacturer’s recommendations.
4. Each person you ask may be from a different vocational background. A plumber may recommend once a year for a water heater. A boiler operator may recommend once-per-week; etc. The answer depends upon the circumstances of their occupation.
This question is similar to asking how many licks you may take of a lollipop. – It depends on the manufacturer and the circumstances.