TMJ has returned 3 months after 25 units of Botox to masseter muscle. How long until the masseter muscles begin to reappear?

Neuromodulator treatments like Botox and Dysport are temporary lasting 3-4 months on the upper face and 4-6 months for the masseters. The treatments need to be repeated to maintain results. Since your results lasted 3 months, you need to get your masseters injected with Botox every 3 months or 4 times a year. 

The fact that your TMJ has returned 3 months following the treatment using Botox, indicates that your body breaks down Botox within the masseter muscle in approximately 3 months. The masseter muscle does not disappear and return, it's simply retains its strength after the botox wears off. You should put yourself in a regular schedule of Botox injections every 3 months and I'm sure this will help you a great deal. Good luck from New York City