Average Length of Time Botox Lasts?

I've had Botox and Dysport once each since last January in the glabellar lines and forehead lines. It seems that after 8 weeks much of the movement is returning. I am 48, eat healthy, cardio/toning exercise daily, have great skin (just deep ugly frown lines :( i know results can vary for each individual, but it seems like mine wear off a lot sooner than time to return for treatment. I know the ratio is mixed good, the first initial weeks - it's great. Is this the norm?