This is a late reply, but as someone who has done Botox for a long time I can advise that I learned through experience that for me, small doses more often yields the best results. I did get too much once and ended up looking very tired with a too heavy brow. It didn't last for the full three months - more like 3-4 weeks before it began to improve. Find a practitioner who is comfortable working with you to find the minimum dose required for a good result, even I it means you are going in more often for touch ups. I would not try to correct with more Botox.
That was a minimal dose! I never lost any movement in my forehead at all. The only indication that I'd had Botox was the heavy lids for about a week... then all evidence of Botox disappeared.
My first experience with Botox was a tremendous disappointment but I will be trying again in a few weeks time (with a different practitioner, of course).