When Does Jaw/masseter Botox Wear Off?

This greatly depends on the amount of units that are used and the mixture of the toxin. Most toxins like Botox usually last from between 3-4 months.

If you liked a the fuller look of your face, it's not clear why you would have had this treatment. Depending on how many units were injected, the muscles have had time to atrophy and will need time to build up strength again.

Injecting Botox into the muscle weakens it, and over time while the muscle is weaker it atrophies (decreases in size), providing the slimming effect. The effects of Botox to the masseter muscle last an average of 6 months. Once the effects wear off, the muscle will begin to function more frequently and regain its strength, slowly increasing in size again. How quickly it regains it's strength and size varies patient to patient and also depends on the dose of Botox injected. 
I've created a video where I review treating the masseter muscles with Botox, I hope this helps in your research!