At a rate of around 4.5 ppm per 1 ppm of ammonia that the fish produce.
It very much depends on the set-up you have, your tap water nitrate and what you personally are happy with. I normally recommend 25 ppm as top because that's the lowest maximum you are likely to come across in any aquarium species, but most aquarium fish can survive with much higher nitrate (some even into the 500 ppm area). Nitrates do have *some* negative effect on fish, but how much of an effect and at which quantities varies from species to species and, to be honest, the research is very lacking in this area.
On the other hand, my tap water nitrate is 0 ppm, and without water changes, my planted display tank nitrate is around 5-10 ppm, so I end up not doing as many as many water changes as I do in my Mbuna tank (which has very *ahem* few plants) just to maintain the nitrate level for the plants.
The most effective way to reduce high nitrates with water changes is to do *large* water changes, but this is not always possible if there is a pH and water hardness difference between tap and tank water.
