It varies by how dirty your fuel is. Tips: Transmission fluid is the cleanest; diesel truck & equipment oil is the dirtiest. The average is 1-2 times per season.

Pre-filter your oil prior to pouring in the tank. ( A mesh screen will help ). Do not mix oil with solvents and parts washer fluid. Do not burn gear lubes. They will cause more maintenance even if mixed with fuel oil. Clean filters monthly. Drain water / antifreeze from tank. Vacuum combustion chamger 1-2 times per season.

Annual maintenance according to what your manual instructs is necessary.