Botox - when does it last longer, when its your first time or when you have done it alot of times?

The information you have assimilated is correct in the sense that the first injection may be insufficient and the dosage must be tweaked to create the difference you expect.  It usually takes between 2-14 days for the first injection to take effect. Your board certified plastic surgeon may schedule you for a follow up at the end of a month to assess if the anticipated result is as expected or if dosages need to be adjusted. 
The wrinkle softening effect of Botox is temporary as the toxin is broken down by enzymes in the body and eliminated over time.  Botox injections typically last from 3-6 months before the effects are no longer seen. Muscles become weaker with more Botox injections and do not require the initial jump start.  Every patient is different so you will have to learn the spacing that works best for you.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

I find that in patients who get Botox for the first time, the results only last for 3-4 months. But patients who get Botox regularly can achieve results last up to 6 months. Hope this helps.  

Generally the first time one does Botox, unit per unit, it will not last as long as subsequent administrations.  The muscles will not have fully recovered if it is injected on a regularly schedule.  

There is no way to predict how long your Botox will last.  On average, it will last 3-4 months.  After some injections it may take effect sooner, and sometimes it may take effect later.  After other injections, it may last a shorter period of time or it may last longer.  I have seen people who have done it for years and it will last 6-10 months in them.  I have seen other people who have done it for years and it will last only 2 months, presumably due to the antibodies that occur due to the structural proteins that it is manufactured with.  First time, 20th time, there is no way to accurately know exactly how long it will last.  Positive for you, it will go away, just be patient.

great question because both are right. Some patients get better results with repeat because the muscles get weaker but some patients get results not so good because  they are making a lot of blocking antibodies but it cannot be predicted which might occur

My opinion is that it does just the opposite. The more you get it, the more time you can go between treatments. I think its because you "frown " less while the Botox is there, which makes the frown muscle grow smaller as it is less active.In my practice, I still consider Botox my "home run" in that it nearly always gives me a satisfied patient!Hope this helps you,Dr Beverly Johnson