i say theyre still always effective.
why? because surely you measure effectiveness as whether your hair would be better off or the same not using them.
of course, if you drop minoxidil or finasteride, at any point, everyone knows youre in for a big shed. which shows us that, even though 10 years or whatever down the line, they may not be growing more hair, they are still guarding against some dht (finasteride) or maintaining the hair you regrew (minoxidil).
if you put it in these terms, of course theyre still effective, because if you dropped them your hair would be worse off.
ps: i still think if you used minoxidil for 20years, and you saw now regrowth after year 2/3 or whatever, your hair will still be better than if youd never used it before. same with finasteride.