What you want to do here is take a reading of your nitrates and phosphates. Expect them to be non-zero after a good initial cycle. Base your water change or changes on those numbers. You want to get them low but not all the way down to 0. The amount of water changing you'll need could range from none at all to close to 100%. For a very large water change, you usually want to do it as a series of smaller ones done several days to a week apart.