Never! :P Or, when it gets really, really dirty after like a year or two, haha.
You have a filter and a tank large enough to be cycled, so let the bacteria grow and do its work so you can stick to weekly 50% changes after your fish recovers and the tank cycles. Do vacuum the gravel once a week, though! If you have live plants, then you can stick to vacuuming the gravel once a month or so.
The easiest way to make sure everything is going well while cycling is to use a liquid test kit. Look up the freshwater aquarium nitrogen cycle.