When do I have to do a 20% water change for my 35 gallon fish tank? EXP. FISH

[fanmegaman]

TANK OWNERS ONLY!? Ok I have 2 two sailfin or yucatan mollies, one male betta fish:note that the betta fish is not aggressive with the rest of his tank mates) and one pleco I think but hes a bottom feeder sucker fish. So when do I have to do a water change? Am I forgetting to add something else if I do a water change? Because I add Prime: a stress coating an anti-chlorine for the water. So tell me EXPERIENCED FISH TANK OWNERS ONLY!