How long the shot lasts varies, when I first started doing it I wanted another treatment in about 3-4 months....after awhile it starts lasting longer because those muscles atrophy. Now I go every 5-6 months. The shots are very easy to inject, it doesn't take a whole lot of skill, so you can pretty much shop for price and go to the Dr. with the best price.
I would squint in the sun between my eyebrows in my twenties and would have definitely done it in my twenties. I am 41 and I have no wrinkles on my forehead or between my eyebrows at all because I started soon enough.