Good question, lots of variables to truly have a definitive answer, I guess it would vary on engine hours / mileage + rpm average, quality of fuel, if using fuel additives, etc. If you have the 8.3 (not sure if you have the NHC 250) I've read that the little filter can be 2 years / 22.5k miles, but some guys do it annual or at 1200 engine hours. I think the little filter is cheap enough to do it more often, but that's just me lol.