Nitrites become nitrates if you have the proper bacteria and water conditions present. Once you have a properly cycled and balance water cycle, you should never register nitrite readings.
Nitrates are different as they will remain in the tank until they are removed by water changes, plants which you then cull or discard, materials (like resins) that adsorb nitrate and so forth.
If your question is about resin media in the sump that adsorbs the nitrate, then as others have stated, it will be adsorbed at a relatively constant rate as long as the resins are capable (not saturated.) Once they are saturated, they will cease adsorbing.