Well this more or less depends on the riding conditions. If it is dusty or pretty muddy I would recommend changing the filter after every single ride without exception, especially if you are riding on a track or out on the trail with a group of other riders who are kicking up roost and dust.. However if it is not too dusty or muddy you probably can get away with every other ride. If you plan to ride more than once between filter cleanings then you should always atleast give it a visual inspection after a ride to make sure it doesnt look too dirty. I personally clean my air filter after every single ride regardless of the conditions because I wash my bike after every ride, usually with a powerwasher, and that causes the filter to get wet and absorb water. You want to avoid running the bike with a wet air filter because that will allow the intake to suck in moisture causing further problems. At the very least remove the filter after washing to assist in the drying process and re-oil it to ensure it remains tacky. The job is not difficult but as we all know it is probably one of the least desireable maintanence tasks. What it comes down to is that the air filter is essentially the life of your engine, keep it clean and it will save you loads of hassel in the long run. In my opinion it is worth the little bit of time and hassel to clean it after every ride.