Check it after each summer. Since you live in a place where there's lots of dust during the summer, and you off road a lot, you'll probably need one more often than the average owner.
Supposedly, you can use the little valve on the intake tube to check and see if the filter is operating correctly, but I've never seen mine work.
Take it out and shake it or tap it on the ground, if a bunch of dust falls out and you can't get it to stop, then it's probably time for a new one. I've seen mine with about 2 cups of dirt fall out of it. There will always be some dust in them, but too much is bad. I wouldn't recommend using an airhose to clean it out either. Air filters donot respond well to blowing them out, and it usually doesn't help for very long.
You can always replace it with an aftermarket filter that you can clean and reuse. I don't think they come in Yellow though.
How many leaves were in there?
