It's simple. If your tank is cycled, you can do 10-25% water changes weekly once it's finished it's cycle and it's stable. You should still be testing the water weekly with a liquid test kit to make sure you're on top of things and you know what your parameters are.
If your tank is not cycled you should be doing 100% water changes at least weekly or more often if your tank is small (2.5 gallons or less). You can throw a 50% in there if you're pressed for time but the smaller the tank, the more often you should be changing 100% of the water. A test kit will help you know when your ammonia is too high. It's important to have your own liquid test kit or it's just a guessing game. Fish pee and poop all the time and that's where your ammonia comes from so it's constantly being introduced to the tank. That's why you have to change the water as much as you can in a tank that's not cycled.
I hope that makes sense.