Are you going to have a sump or go sumpless? How about a protein skimmer? Are you going to run carbon and/or GFO? Ideally, you should do a 20% WC every 2 weeks. Especially if you're going sumpless. But IMO/IME you can stretch it out to once a month if you have a sump, protein skimmer, run carbon, etc. Also, a lot has to do with your bioload. Light bioload = less frequent WCs required, but as time goes on and your bioload increases, you'll have to do more frequent WCs.