The Masseter muscle, as well as other facial muscles, may maintain good outcomes for a longer duration with repeated treatments. However, only time and one's one pattern of animation and muscle contraction will determine how long they will maintain results without treatments. Ideally, once receiving optimal results for the debulking of the Masseter muscle, a maintenance treatment once or twice a year may prove valuable. The below link will lead to before/after photos and articles on this service.
The idea behind Botox is that the more times you are treated, the more the muscles atrophy from inactivity, and therefore the muscle takes longer to "bulk" back up. In most cases your muscle will regain full function and return to original size when treatments are permanently stopped. Kindest regards Neil J. Zemmel