I personally have used it for almost 2 decades (on and off) but I don't think you "have" to. It isn't like the results will reverse themselves. I have very oily skin and live in a very hot climate. My face glows with oil. People used to ask if I was hot and uncomfortable and I had no clue why they were asking - well, it was because I had a wonderful oil sheen on my face. The Retin-A was to decrease the oil and it totally has. I just kept using it due to the OTHER benefits I received (no breakouts, no blackheads, no oil, wrinkles reduced, fades discolorations, really clear, creamy complexion).
I previously used a lesser concentration and used the cream but never saw results. It wasn't until I used the Retin-A 0.1% microgel that I immediately saw benefits. Again, I use it every night but I know some people are only able to handle it every other night or so. The thing to remember is you only need a pea-sized amount (any more and it just causes pilling - won't rub in and you are wasting it) and to not put it in creases (bottom sides of nose or chin crease) because it seems to cause redness there. Sometimes it is recommended to wait 20 min after washing before applying to help decrease sensitivity but I put it on immediately after washing without problems.
To the original poster - I don't think the Retin-A helped my melasma since it literally one day just disappeared. I am not sure what caused the sudden onset or sudden disappearance. But I know others have used it for melasma, scars, and to fade age spots.
. I wasn't as careful about sunscreen in my teens/early twenties as I am now, but that doesn't change what's already there....