For those of you who have had Botox, how long does the effect last?

I feel like my jaw muscles have developed too much lately, so I'm considering getting jaw Botox.

I have a habit of clenching my teeth, and I often wake up in the morning with a stiff jaw. People around me suggest I try it, but I'm hesitating because I've heard the effects don't last as long as expected...

Since it's my first time getting hit, I came to leave a message because I have a few questions-!!

1. When do you usually start to feel the effects of jaw Botox?
2. How many months did it actually last?
3. What is the best dosage to start with when getting the first injection?

I would appreciate it if you could also let me know if the satisfaction level was very high and what the cycle was like thereafter!! :)