Upstate, I think what type of soap you use to wash your car with, how many times you wash your car, acid rain, the suns intensity, gifts from birds and insects, and what kind of washing tools you use to "scrub" and dry the surface with, such as, a furry mitt, brush, cotton towel, chamois, etc. would all have something to do with how long any one particular car wax would last over another.
I read where using a chamois actually "strips" wax off your car over time with repeated use. But I also believe it's probably one of the softest materials to use to dry it also (next to blow drying that is).
I'm sure Aero Blue, or Andrew can give you the best info on the right cleaning products.