I just pull my fanfold filter out and look at it. Plus, I replace every 2 years worst case. I typically run it for 5-6 hours/day max for 4ish months out of the year, so that's 600 hours/year. Although I have to admit, I just replaced it after nearly 3 years/1800hours cuz the dude was out servicing it so I just had him do it. He looked at it and said that it didn't really need to be replaced, but as long as he was here...
It's only $40, so I suppose I could replace it more often. It's just such a boring activity though.