For how long can a pulpectomy last before the need for a root canal?

RCT. Risks start as soon as the pulpectomy is finished. Normally it's a good idea to follow up with root canal therapy in about a week, especially if there was an infection present or in the works. When will the volcano erupt? Technically the need for rct is initiated when you do a pulpectomy.
It varies. That is kind of a difficult question to answer because there are a lot of factors involved. I wouldn't recommend waiting any longer than one month before having the definitive root canal done.