While cycling a tank i would test for ammonia and nitrites every other day until they disapeer so that i know exactly how far i am in the cycle.
Once a tank is cycled then you can test just once a week and after the tank has matured (over 6 months old) you can cut out testing for ammonia altogether, although you should still test weekly for nitrites and nitrates.
In very well established aquariums (those that have run for a year at least with no problems) you only need to test for nitrate once every one or two weeks depending on the stocking levels.
