About 4 months. I can then see the lines again I have mine done every 5 months. I only have one vial though. I think its about the amount you put in rather than the strength of the botox as such. My practitioner also puts most of it in and then I go back two weeks later for her to check what it has done and she then finishes it off. That way I only ever get a very natural look.
I have had two vials before but it made my brow feel heavy.