IMO; How much and how ofter you change water depends on the bioload,filtering, anarobic bacteria,feeding etc.
I do 35% once a month but I have only small fish in a 75g. My fish are healthy so I can justify this routine. If all of a sudden you start to have sick fish (IMO) More/larger WC would be apropriate. Don T.
P.S. Consider closing all your windows and not flushing the toilet, How long would you want to go without a WC?