Every two weeks. Weather might push it out a week to the next weekend. Every other wash gets a wax afterwards. Different rules for winter: once a month, or soon after my car gets salted. Ends up being around two weeks on average, probably.
Per google:
MythBusters found that a clean car gets better gas mileage than a dirty car. In their experiment, a dirty car got 24 mpg, while a clean car got 26.4 mpg. This was due to the clean car having a smoother, more aerodynamic surface, allowing air to flow more efficiently.
Surprised this hasn't been mentioned yet. I'm a low-mileage driver and I am not worried about mpg. I wash more out of habit, back from when I 'loved' my cars.