I clean mine when I notice it's dusty. Sometimes, like Lighthouse, that's when I have the cover open. I use a handheld vacuum cleaner, which is easier to maneuver around the system. I clean up all the dust on top of the case and on the surrounding area since that could be sucked in next. First to clean are the *intake* vents because cleaning the exhaust first can draw the dust from the intakes further in. Then clean the exhaust fans and the power supply exhaust. Open the case and vacuum all the dust possible. Then it's time for blowing air.
The best way to blow out the inside of the case is outside with a Shop-Vac set to blow. But, for cleaning inside I do use an air duster can. When using the air duster to blow the dust, I also use the handheld vacuum to take in the dust which will just settle back down if not captured.
"It's Mega Maid. She's gone from suck to blow!"