For masseters, as Dr Wong has pointed out, Botox leads to a reduction/atrophy of the jaw muscle. However, the results can vary depending on dosage used vs the number/location of botox injections.
Only when it diffuses into the muscles will you have a stiffness of the face due to Risorius and other muscle involvement.
It will last between 6-12 months depending on lifestyle behaviour and the duration increases over the sessions done.