with our schedule its more once a week. some of the smaller thanks get more though cuse its less of hassle and not a two person job but the bigger tanks needs to people and we are not alwas able to do it 2-3 a week.
Depends on the tank for me, my 150G and 100G both get 30% waterchanges twice a week. My 55G and 15G I dont suspect will need waterchanges more than twice a month due to live plants and lower stock.
My poor 15 is so overstocked(Mom and lil sis's tank actually) I change 3 gallons a day with a gravel cleaner. Is this detrimental to the fish it seems invasive to move their stuff around to vacuum daily