I've read for many it continues to work indefinitely. However, the benefits of propecia only lasted about 4 years for me. I was initially a great responder and I grew back about 90% of the crown hair that I had lost.
But it slowly lost its effectiveness and after 4 years, I had lost all I had gained. I stopped taking it and there was no difference even about a year later, so the drug had done all it could do for me.