How often do you all get Botox treatments between your eyebrows?

I feel like the wrinkles between my eyebrows are deepening when I frown, so I plan to get Botox injections regularly. When do the effects of frown line Botox usually start to appear, and when does it look most natural? Also, since I will need to get injections again when the effects wear off, I am curious how often people typically visit the clinic. I am worried because I heard that getting injections too frequently can lead to resistance.