How long does the effect of the Botox I got for stiff jaw muscles last?

I have a habit of clenching my teeth while sleeping, so when I wake up in the morning, both of my jaw muscles are always stiff and sore.

My face shape seems to be getting more angular, so I booked an appointment to get Botox for the first time in my life, as it is said to be the best for loosening tense muscles.

I heard that since it isn't permanent, I have to get it done again before my muscles grow back to maintain a slim jawline.

Although it varies from person to person depending on muscle mass and chewing habits, I am curious about the average duration of how long the slimmed-down jawline usually lasts after a single injection of Botox.

Please let me know if avoiding tough foods makes the effects of Botox last a little longer, and whether the effects tend to last longer if I get pure toxin. If I need to get it regularly, I am wondering how many times a year would be appropriate.