It says that Propecia or finasteride takes about 3-12 months for it to actually work. How long does it maintain your hair on average though if it is successful?
It depends on the individual. In the 5 year study, the haircounts were still well above baseline after 5 years. So if you're an average responder, you'll maintain for at least 5 years, but probably a few more beyond that.
Here's a link to a graph that shows the haircounts over the 5 years: