You know, it's up to you. If you have a low bioload and your parameters look good, you can get by with once a month water changes. There are people in my reef club that run their tanks stocked very lightly and therefore feed very lightly and do water changes once every 4 months, and the tanks look incredible. If you have anything more than a couple fish though, I'd not recommend it. You could always try it and see what happens, like I did. I went without water changes for 6 or 7 months, only topping off for evaporation. It didn't work :(.