A good indicator is when you wash the car. My windows basically dry themselves when I use the pooling rinse or "sheeting" method. Keep in mind the sealant on the windshield will degrade faster than the rest, due to the action from the wipers, using wiper fluid, etc. I do my windows twice a year (say, april and october) and the windshield will get an extra one or two applications to maintain the protection (april, july, october, and weather permitting, january)