I don't actually sit there with a can of compressed air and make sure I get every big of dust out, but I do try to make sure it's relatively clean every so often...
about once a month, dust is a problem in my house, dont know why.... and compressed air is so cool, i love that stuff, especially if you turn it upside down, lol
I usually only do it when i'm changing something in it...last time was 2weeks ago when put more RAM in but before that was prob 3months. Not much dust floating around in my room.
My place is wicked dusty, so I had to home-brew some filters for the case intake fan and the CPU fan duct. I used air conditioner foam to make them. I clean those like every three months, but the inside of the case gets blown out with canned air every six months or so.
Tom