You may be already done (depending on how much ammonia you are adding). When you say the readings are back to 0 in a day are you just talking about ammonia, or both ammonia and nitrIte?
Your high levels of nitrAtes tell me that you have already accumulated a large amount of nitrIte to nitrAte converting bacteria. It's possible you just need to do a large PWC
I would recommend (if both ammonia and nitrIte are going to 0 in under 24hours) a large 80% PWC
or so followed by a single 2ppm ammonia dosing. If you detect only nitrAte after 24 hours your good to go.
What often happens when ammonia dosing for a fishless cycle is that you build up a large amount of nitrIte in the tank. Even if you have enough nitrIte to nitrAte converting bacteria present, you have all this built up nitrIte to get converted. It falsely makes you think your cycle is still going, when in fact, you might be able to stock very shortly.