i tend to overstock a bit so minimum 50% weekly . sometimes 2 or 3 times a week depending on how busy i am or how much i've been feeding .
i have a community tank with tetras and such ...that one gets less maybe around 40% every week or two ....
We do a 20% WC every week on our bare bottom tanks and koi ponds here in the tropics. The koi ponds have a constant water drip. We siphon all the untouched food left in the tanks 1 to 2 hours after feeding and top off the water level every day.
I do 50% wc's weekly. In general one week is simple 50% siphoning and replacing water, while the next week is a full gravel vacum of 50% and water replacement.