I have not done a water change in over a year. Before then, it was about 2x a year for a couple of years. Basically I have changed my water apprx 5-6x in 5 years (about 20% each change). I also have stopped dosing kalk and now only dose rodi topoff. I don't dose anything else but do think it is time to maybe do a 5% change. I have roughly 60% sps, 30% lps, and 10% softies. For fish, 2 maroons, 2 chromis, 1 vlamingi, 1 kole, 1 yellow tang, 1 orange spotted goby, 1 yellowtail blue damsel, 1 cleaner shrimp, 12-14 various snails, 6-10 various hermits and one huge Derasa. I have not added any livestock in over 18 months. My tank specs are in my sig.
I've done this for two reasons. First I believe once a tank is established, and I mean really established, it's best to keep your hands out of it and not fiddle with changing any system components unless absolutely necessary (like lights for example). Second, I started down this road cause I found myself getting a little lazy doing small weekly water changes. Over the years, as w/c became less frequent, my tank continued to look/act healthy with corals colorful and growing. In fact, I have to frag a couple colonies severely back every six months or so because of growth.
Granted, I have had some Oh Sh** moments like when my 12" Derasa spawned and my skimmer went crazy that I *should* do a w/c. I still didn't do a w/c but I did run carbon for a couple of days when that happened.