I started getting Botox when I was about 45 (I'm a very young-looking 56 now). In my case, I found that the effect was cumulative. The first time I had my forehead done it lasted approximately 5 months. The next treatment, one year. Perhaps I was lucky(?) When I start to see the deep frown line I have between my eyebrows start to show, it's time to call my doctor.
One effect I noticed the first two times was what I call the "angry eyebrow" look - my forehead was very smooth, the deep frown line was gone (aided by a little filler), but the inside of each eyebrow dipped giving me a strange look I wasn't happy with. I started asking specifically NOT to have this look. The areas of injection were changed and I've been pleased with the results since.