Most manufacturers recommend once a month. Always complete tests while no one is in the pool. Test the receptacle or faceplate type by pressing the button marked "Test." The button marked "Reset" should pop out, and the circuit de-energize. Verify that the lights will no longer operate. Push the "Reset" button until it locks down, and verify that the circuit has been restored.
If you have the circuit breaker type, push the test button on the breaker. The handle should move to the center (tripped) position. If so, verify that the lights will not operate. If so, push the handle the rest of the way to the off position. (You will usually feel it catch.) Then press it all the way to on. Verify that lights now operate.
If the above tests fail in any way, your GFCI may not be protecting the light circuit. Do not use the pool until the light system has been tested and repaired, if necessary, by a qualified electrician.
If the GFCI passes this test, it is operating properly.
Please document GFCI tests conducted in the "Remarks" column on your monthly log sheet.