Developing botox resistance? After how many uses?

I normally do botox for hands every 2 years due to cost/pain/etc; done it three times in the last 5-6 years.

Also 1-2x year I do it for teeth clenching. For the last 3-4 years.

I believe I may have developed resistance as my last hand treatment didn’t work. The Dr offered to re do it in a few months with a different brand at no cost.

How long did it take you to develop resistance? Is it to all brands? How do you test resistance to different brands?