One really can't change water too often, but most of us find once a week to be enough--by which I mean, we don't want to do more.
So a once weekly change of say 50% of the tank is good. If you want to do it twice a week, even better, but change more than 25% each time. Two 25% changes do not have the same effect as one 50%. More frequent water changes will mean more stable water parameters and biology, providing the "fresh" water is close to the tank in GH and pH. Just make sure you stay with whatever schedule you start, as consistency is important.