Moe,
I have approximately 1.5" of sand. Though my tank is only a 20 gallon high (24/12/16), I have never seen my nitrates, nitrides and ammonia rise above zero in the 8 months the tank has been up. I have at least 25 lbs of LR and do a 10% water change weekly. I'm not sure if the weekly water change is required to keep everything at zero since I've never done otherwise.
So does this mean that you're finally setting up your tank?
Steve