I would say it depends on the type you have. If you have the one with the clear bottom and you can see HOW MUCH water your separator is filtering then it should be common sense.
I think you enter tournys so run more then most so at a MIN. every 6 months.
If you have the cheap type like stated, $10 ( LOL) then do it every 2 months if you run your boat like I think you would. Heck, you will push 50 hours own your engines per Tourny if you count the pre-fishing.
If I'm not mistaken it cost me $28 per filter for the ones I use, then again my boat has not been on the water in 2 years come May.
One way to tell also is to remove you filter or separator and pour the contents into a MASON jar and let it settle. You will see how much water is in the filter and you make the decision if you should replace it.