Denitrification is a process that occurs in an anoxic substrate. We don't want a completely anoxic substrate, we want to encourage plants to pump O2 to their roots and allow aerobic processies to occur. The constant interplay between a aerobic and anaerobic processies in the substrate hinders denitrification because the bacteria that actually do the denitrification don't like O2 all that much. Hence, while denitrification does occur, it is of very limited value in a planted tank.
NO3=>N2 is not going to be a large component in a planted tank.
As mentioned by Sean, O2 is going to change things for the substrate.
You can measure the amount of NO3 removed by this process by removing the plants and then measuring the NO3 loss without fish/critters etc by several doses of KNO3 to know values.
Then you can get the rate. I did this in the past with a flourite sub at 4 " deep. I did not find much decline over the course of a week, perhaps 2-4ppm.
Other substrates might perform better(higher rate).
If NO3 export is your goal and the plants in the tank cannot do the job enough, lower the fish load etc, or add more plants in a filtration section, they will remove far more NO3 than any anaerobic/anoxic bacteria will, aerobic organisms are about 18X more efficent at that than any DBS/substrate process.
plants alone should denitirfy to a great extent if thriving. the use of a deep sand bed or DSB is old news in the SW worls and works well. it takes two weeks for results at least in a marine tank. again the roots will sereate the substrate so it defeats the DSB alot of time the more plants the less it will matter. most planted substrate depths are around 3 or 4 inches which is the same as a DSB. ultimately it matters little IME. my suggestion is rather to make a refugium and reverse it lighting cycle so it's lit when the main tank is off, both should be heavily planted tis is IMO an almost perfect cycle as plants reverse at night. you get constant o2 from the plants in the light while the plants in the dark create co2 and everyone feeds eachother. its far more beneficial especially if you have fish than any other method of co2 introduction, dsb's, denitrification, chemicals, etc