Roughly 3-4 times per month, depending on if it rains (ha, funny.. I'm in Phoenix) and how much dust is blowing around. Also depends on the birds. Usually works out to once a week or so. The heat also comes into play. Really sucks washing the car when it's 105F out and the sun's beating down on you, so if it's just dusty, I might skip a week or two. Also, sometimes I'll skip a wash and do a 'waterless wash' instead, if it's just dusty.
The past few weeks, my car's been getting bird bombed regularly, so I've had to do a couple extra washes.