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! 

but the good news is that the effects will wear off soon...overall the effects of Botox when used for masseter muscle reduction last about one year depending on how many units were used and how big the muscle was...and when the effects wear off, the muscle only slowly grows to its former size...but never fear that will ultimately happen...while many people want the more angular face, nothing wrong with fuller features...really a matter of personal taste...nothing cast in stone what's best...

The effect of Botox for the masseter muscles can take one year or longer to wear off.  Chewing will help strengthen your masseter muscles so you could try chewing gum unless your dentist has warned against it. 

I'm slightly confused as to why you did Botox injections to the jaw muscle if you prefer a rounder face; Botox to the jawline helps narrow the face. Nevertheless, Botox in the jawline usually lasts between 6-12 months, depending on how strong the muscle was that was being injected. Remember too that because of the Botox you haven't been able to move that muscle the same way, so it has atrophied a bit, and it won't just bounce back to what it was before. You will need to spend a bit of time after the Botox wears off, reusing that muscle and restrengthening it. Then it should return to what it was before.