An interesting study, amongst many others, has been done regarding the length of time Botulinum Toxins can actually last. In my experience, if you are very regular with your treatments, you can slowly, slowly lengthen out the time between treatments. So starting with 3.5 months apart treatments, then inching them out to 4 monthly then slowly longer and longer. I have been treated with both Dysport and now find after about 4 years, I have been able to do my treatments, 5 – 7 months apart.
Check out this link below, to see their study.