How often would you do water changes?

I have a 40 gallon breeder sized tank and the parameters are 0 ammonia, 0 Nitrite and 0 Nitrate. Yes it is cycled. It's planted heavily so that's the reason there's no Nitrates. I dose with Flourish, Flourish Excel, and Flourish Nitrogen. The Ph is a steady 6.7 the temp is 80. I do top ups and a water change one a month of about 25%. Do I really need water changes more often that that or at all given the levels? And I'm sure the nutrients nitrogen and carbon are getting used up by the plants as there's quite a few of them and rapid growth.