That is very subjective; it varies based on how often you wash your vehicle, what you use to wash your vehicle, whether or not it is garaged, what weather / dust / pollution it is exposed to, etc.
In most cases, it will bead water for 8-12 weeks. Some people report up to six months of water beading, some see less than 8 weeks. For best results, wash your vehicle on a regular basis with a gentle soap so as to not strip the wax off. Baby shampoo works great on lightly soiled vehicles. Use about 1 tablespoon of baby shampoo for every gallon of warm water.
You can also add one tablespoon of CSI Q-7 Wax to about 4-5 gallons of warm soapy water to extend time between waxings.