pag kailangan na iwax, then rewax... there's no such thing as a time interval guide coz it all depends on the following factors:

How often you wash
Where you use the car
Where the car is parked
Exposure to Contaminants
Weather
Type of wax/sealant used

Too much factors to consider... It's not correct nor logical to give a timeframe of waxing... or washing for that matter... People who works/lives in clean areas and parks indoors most of the time can stretch these intervals much longer than a car in the Metro that parks outdoors, etc etc etc....

does rain remove wax... even if you don't touch the car, air, moisture, heat and humidity will reduce wax somewhat...