A couple hours to overnight. Testing really is only needed prior to a water change. *Please* let me clarify. If you know what you are adding back in or taking out (be it nitrAte/ammonia/ferts for plants), after a water change unless you made a gross error or performed a test improperly the results of that test should be very close to what you should have calculated.
Let's say you have 1.0ppm ammonia (cycling tank) and want to get the ammonia down to 0.5ppm or less. You should know right away that a 50% water change is the LEAST amount of water you have to change to get close to that value. Any less and the ammonia is higher, any more and you can calculate that as well.
Same with dosing ferts. Say your water is very soft and you want to increase your carbonate hardness (KH). You should know beforehand how much baking soda is needed before doing the change and so you can be confident that a water change will put the tank exactly where you need it.