How would I know if/when I need more Botox in my masseters?

 Botox in your masseter muscles should not affect chewing, so don't try to use that as a sign for whether you need another treatment or not. Botox in the jaw can last up to six months or more, so I would let that serve as a guideline. Every person is different, and so you will have to pay attention to how your face looks. You may find the effects wear off after about 4 months, or they may last til almost a year. A safe bet is to re-treat biannually. You should be able to maintain good results without spending too much time in the dermatologist's office. Consulting your board-certified dermatologist will allow the two of you to develop a schedule for your Botox maintenance.

I would suggest that you schedule a maintenance visit in 6 months. This is when most patients find that they need a touch up. Hope this helps. 

Botox and other neurotoxins are used on the masseters to slim the face.  Treatment should not affect  chewing.  For the initial treatment, you may require more than 20 units per side to see the desired results.  However, once the desired results are seen you will typically require less for maintenance and less frequently (every 6 months) than for other areas on the face.  Consult with a board-certified and experienced cosmetic physician.