I go 5-6 months at a time without changing my water in my 75g SPS dominated tank. I dose Cal, Alk, (and trace elements when I remember) feed 6 fish heavily, and broadcast feed the corals twice a week. I do the same for my 57g Softie/LPS tank.
I test the water monthly, clean the skimmer completely (total tear down) once a month, and change filter socks (25 Micron) every 2-3 days. I run Phosban and have a fuge with chaeto.
I have a close friend who does the same on his tanks.
Every tank is going to be different. Your fish and corals will let you know if something is off. You will learn as time goes by how your tank looks at it's best, and if things are not looking good.
If I see something not right in my tank, I will do a WC before the usual time, but it's rare that I have to do that.