It's heavily dependent upon your original male pattern baldness case (your genetic susceptibilty, not just how far it was allowed to progress). Guys will less severe cases of male pattern baldness in the first place might maintain indefinitely on the current meds. The worst early-balders are sometimes complaining that they can't even stop their loss at all.
And then there's the side effects issue.
I could probably halt my male pattern baldness completely and regrow a whole Norwood level for at least a couple of years with about 2mg of finasteride per day. But that's never gonna happen because the sides get way too severe at anything over 1/2mg.