Seriously now. About every 3 months.
One thing I do that I encourage to others is, if their desktop arrangement places the computer tower on the floor, to get a "stand" of some kind to raise it off the floor 4-6 inches. This will minimize the amount of floor level "dust bunnies" that get sucked up by the intake fans/vents.
A nice "antique" wood packing box 9"w x 24"l x 6"h works very well for me personally (watch for them at yard-sales/flea-markets).