For both my 85 Redpra & the 81 Silver Slug, it varies depending on the time of the year.
Mid October - Mid March or possibly Mid April, depending on the weather.
Around the beginning of October, each car get's a final protective coat of wax (the Redpra just got it's coat in Vegas, need to do the Slug pretty soon), THEN for the next 6 months, it does not get washed AT ALL (oh the HORROR!!!!!! :shock:

). Unfortunately, both of these are my daily drivers, and up here in the PacNW (at least in WA State), this time of the year becomes the crappy weather/rainy season. For me, it's pointless to wash my car for the next 6 months when it's raining outside every day, or at least every other day (or every 2 days). And when it's not raining, then it's pretty damn COLD (low 40's - high 30's), which means that it's near freezing at night, thus the outside water valve's are shut off for the next few months. If I'm really desperate, I'll go to Elephant Car wash, but they usually do a pretty crappy job. And I hate those DIY car wash's w/ the spray gun hoses w/ their harsh water that leaves water spots (yes I do use the soft water setting, but still)... More work for me to do in the spring.... Plus again, it's cold outside....
Mid April - Mid October. Both cars get washed usually every 2 - 3 weeks. Usually less if there is a fresh coat of wax on it, I then just dust if off w/ the Cali Duster. If there is allot of dust blowing around, then the Redpra get's the car cover thrown on. Since the Slug is the car that sits out in the elements (the Redpra get's the apartment carport), and our parking area has allot of pine tree's around it, this car get's it's annoying accumulation of pine needles in all of the hard to get to places. Both cars now permanently each have a can of aeresol/compressed air, I just use this to blow all of the pine needles off.
Sometimes, both car's will get washed more frequently, depending if any bird crap, or tree sap is on the paint surface.