“How long does it take for dentures to stop hurting?”

I don't have enough information to properly answer your question. Best to go back to the dentist who made them.

Dentures should not hurt. If they are new, not unlike a new pair of shoes, they can cause sore spots. Your dentist can make a quick and easy adjustment to correct that. If they are old, they may be in need of a reline, as the base bone shrinks over time and the fit of the denture shifts, again causing sore spots. In either case, a visit to the dentist can quickly fix your problem.

It's not supposed to hurt from the beginning.