Hi Matchu,
everyone I ever asked this question of told me the same, once a week is best. how much you choose to change is another question. I do about 15%, some do 10 some 20 Its really up to you. However much you choose I think the weekly change is good because It means you pay regular attention to your tank, the excitement of a new tank inevitably wears off and a regular routine focuses your attention and and gets you familiar with your fishes behaviour and meansyou can nip problems in the bud early on. Best of luck friend.