I'm with the "never" crowd. The only reason it might possibly need any attention at all would be if you shot a whole bunch of .22LR through a conversion kit. However, the conversion kit makers recommend you swap it out and fire a few full power rounds every couple magazines. Reason being, the high pressure from the gas on a .223 round will blow the junk out of there.
If you're going to shoot THAT much .22 through a conversion kit, just get a cheap gas tube, cut it off, and install it in the gas block upside down. Then swap back with a normal tube when you want to go back to rifle rounds. Spike's sells a small tube blank for this purpose, but it costs as much as a normal tube anyway.
If it ever gets clogged, it's a replacement item, not something to clean.