It depends on too many things to actually go over, but really it depends on the needs of your animal.
You want nitrates at least under 20ppm at all times. Do your h2o changes to meet that goal. You will need to at least test for nitrates to determine what works for you and your animals, as you add more animals, or change filtration, you are going to want to closely monitor your nitrates again.
Another thing that helps me is setting up a water change day.
It is just a day you set aside to be h2o change day. There are no excuses, you just wake up in the morning and do it because that is what you always do, and it is automatically part of your plans. Otherwise water changes can get pushed to the end of the priority list, and sometimes they dont happen. I enjoy h2o changes actually, every water change saturday.