If it gets really bad, I'll tear it down once a year when the prime running season is over and wet weather sets in (winter here is usually rainy weather). If I don't get it that dirty, I'll do it every other season.

Don't use a hose, prefer to use a hand sprayer and toothbrush and let it soak to get the bad buildup off. Once I have crud broken down, more hand spraying gets it rinsed away. May take a few cycles of spraying and cleaning to get it clean, but this seems to agree with brgs and etc.