I know from personal experience, and from a few comments I've read in the last month alone, that it's not entirely uncommon for people to end up on a rather low dose after a few years of dealing with such meds.

Usually after rejecting doctor recommended doses and controlling their own dose to min-max with regards to any benefits vs side effects. Standard recommended doses for a lot of these meds are just godawfully high, and it takes a while for some patients to recognize and address this themselves.

There's also a fair bit of variation in efficacy and potency between different generic manufacturers that throws things off a lot too.