Well, the size of the plant, light available, and water flow are going to control how much nitrate are utilized by the pothos. But, I will tell you from my experience they work better than expected.
But, I want to ask a couple of questions about your system. What size tank is this, what is the stock list, and what is the filtration? My reason for asking is you have one of three situations. You are not doing enough water changes (or a big enough water change) to remove nitrates, you have a major source of nitrate in your tank that you aren't taking care of (dead fish, detritus build up, filters need cleaning, etc.) or you are WAY overstocked.
Pothos can help, but there may be a larger issue at hand.