The need for a partial water change always depends upon the fish load in relation to the water and biological equilibrium. The only reason to do a water change is to remove something "bad" and replace the water with fresh, and this is for the good of the fish. There is more than just nitrates involved in changing the water. There was an excellent 2-part article on this in the November and December 2009 issues of TFH, and a few weeks ago this topic came up somewhere on this forum. I won't get into that now, as you asked specifically on keeping nitrates low, so I'll respond as best I can to that question.
Plants will, if in absolute balance with the fish, handle everything and there is literally never a need to change any water. However, this means a moderate fish load and a high plant load, in a suitably sized volume of water where the balance is able to exist.
Diana Walstad advocates a 50% water change maybe once every six months. Other authors say much the same. I still do 50% every week; but I have a heavy fish load, more than what would be considered a natural balance with the plants. In my 115g the nitrate runs 5-10ppm, whereas in the 90g and 70g it is less than 5ppm; there are considerably more fish per volume in the 115g.
Planted aquaria that are balanced have very low nitrate levels, from zero up to 5 or 10 ppm but very rarely higher. As indicated above, the plant and fish load determines this; the more fish and/or fewer plants, whichever, the higher the nitrate. But it is not at all unusual to have a heavily-planted tank, with a balanced fish load, that never has nitrate reading above zero. This is using the API test kit, which admittedly is not absolute accuracy scientifically speaking, but is sufficient for the purposes of most of us.
The reason is that plants use ammonium/ammonia as their preferred source of nitrogen. Fish and bacteria excrete ammonia as a waste product of their metabolism. In acidic water this automatically changes to ammonium--which is non-toxic to fish [this is how most ammonia detoxifying conditioners work, by changing the ammonia to ammonium]--and the plants grab it fast. In basic water the plants have the capability of using the ammonia in two ways I won't go into here, except to say one is to convert it to ammonium that they then assimilate as their nitrogen. In either case, they tend to out-compete the nitrosomonas bacteria which is why the nitrification cycle in planted tanks is minimal. This means very little nitrite results, which in turn means very little nitrate. And there are some plants that will use the nitrate, though most prefer ammonium because the process within the plant to change the nitrate back to ammonium is more labour-intensive and therefore uses up valuable energy.
Hope this helps to answer your question.
Byron.