The transplanted hair can achieve the same length as the hair in the donor area.
The grafts behave like all non-transplanted hair. So they will all grow as fast, and if you do not go to the hairdresser, you will eventually have very long hair.

Transplanted hairs actually possess all the properties of 'normal' hair. This means that if you are gray, the transplants will also age. If you pull your transplants, this will also hurt!
Note: it takes up to 12 months after treatment before the grafts reach their full thickness and density ranges.