Air cans are good, but I don't really like paying a premium for compressed air. I have a little dust-devil vac not the little one but the bigger one with extension and such, and I can reverse the air flow and use that to blast out my computer. I usually do it when I work on it.

Probably every 3-6 months. I would not make a special trip to do it, unless it has been well over a year and depending on how dusty your enviornment is. My home is very dusty, but still wouldn't need to get a thorough dusting more than once a year if it were ignored.