I don’t know how long it takes to know the sides of FIN but is normal to feel different the first month at least. I did it and I know some friends that also felt the same. I think this has something to do with the body's adaptation to the medicine. is rare that you get side effects that last longer than that tho.
let's say you've been using FIN for a year. most likely you will shed a lot when you stop and you will end up where you were supposed to be if you haven’t used the medicine. that is why many people are shocked when they lose a lot of hair and a lot of it does not recover.