Welcome to AA.
Need a few more pieces of information to give you a good answer. What kind of filtration are you using? Is there a protein skimmer? What kind/depth of substrate? What type of fish are in the tank? What else is in the tank? (rock etc) What are your other water tests like? What is the nitrate of your replacement water?
Having said that, with 10 fish in a 55gal, your system is very overstocked and you will likely never see nitrates lower than 20. And only the weekly water changes are keeping them that low. Your aquarium is *likely* done cycling, but only testing the other water parameters will let you know for sure. Depending on your filtration setup, nitrates will probably never dissapear on their own. With a good refugium and/or a protein skimmer you can get nitrate to 0. But it seems to be the most difficult waste to get rid of, and raises in proportion to bioload and feeding. Ammonia and nitrite both should be 0 on a cycled aquarium, with only rare rises above 0.
gl