Estimations vary. I'm on propecia for something like 9 years now and i believe it gradually loses its effectiveness starting year 7. (if it ever was effective, that is)
I'm still taking it though but only 5 days a week.
There is no proof that finasteride looses effect, we just get older, so our hormones don't support us as well in the intervention of drug therapy. finasteride is god, we are but subjects of its wrath.