I'm fairly new to this saltwater tank only about a month and a half... I've got 2 tanks one is 30g and the other 40g how often and how much at a time on adverage should I do a water change...
A lot depends on how heavily your tank is stocked and how heavily you feed, and the quality of your protein skimming, but generally speaking, a 25% WC done on a monthly basis is adequate.
I do small water changes--around 10%--every 2 weeks. For me it is a matter of discipline--when I do water changes, I do other tank maintenance and it keeps me from getting behind.