Once a month. 15 g for a 65 g tank, 40 g in a 300 gal system. 5 gal for a 30 gal tank and 5 gal for a 24 g tank.
Soon I am going to consolidate to a 340 g tank with ~75 gal sump and 50 gal refugium. I will likely change 50 g every month on that system.