hello and welcome to hcm forum,
i would be glad to answer your query,
first of all i , with all due respect to you and your dentist
i would like to inform you that being a dentist we cannot assure you of the time period a tooth takes to moves from its original position to a new position.
you have not mentioned your age here, but i believe you might be older than 20.
ideally, orthodontic treatment is done during the growth years (puberty
but in some cases, we do perform orthodontic treatment beyond that age as well, as in your case orthodontic treatment can be done but the time & the results cannot be decided before.
As the teeth and jaws have developed fully, therefore to retain them in a new position , you will need permanent retainers which hold the teeth in an appropriate place, do not worry these are not visible to anyone.
i would advise you to get the orthodontic treatment done but avoid unrealistic expectations,
just have faith in your dentist.
i wish you good health,