Up until about three months ago, I went over a year without a water change. Unfortunately, the Xenia loved it and took over.
Now that I'm actually taking care of my tank (a 20L), I do 2 gallons every weekend, and I try to fit in another gallon during the week. I'd rather not dose anything if I can help it.
90g AGA, 20g sump (call it 15), 10g fuge w/ chaeto, 5g RDSB (maybe 2 gallons of water?), about 10g every 2 weeks, only 2 spots in the tank with dime sized tufts of HA that won't go away. Undetectable nitrates and phosphates.