I get "toxed" with 50 iu (international units) to brow, glab and crows feet approximately every 4-6 months. My very first treatment with Botox didn't seem to take. I kept waiting the week that they told me it would take to begin seeing results and while parts of my brow felt quite heavy when making expression I never seemed to get the full effect.
I went back shortly after and my PS injected a bit more in the frontalis. I hadn't noticed until she pointed it out, but my Glab lines were gone all the while. Still the brow was stubborn. Most men have a much stronger brow and will require more, but some women are no exception.
For me it was important to allow the botox to help me retrain my muscles. Now that's hard to explain, but I'd watch myself in the mirror and keep myself from making certain movements. When I felt that "heavy" feeling while making certain brow expressions it was an indication that I shouldn't make that movement.
I had my second treatment four months later and ever after it has worked perfectly everytime. Some people with metabolic differences MAY "soak it up" quickly. I have some (serious) body-building friends that required botox every 2 months initially to get it going.
Make sure you know how many units you're getting for the price. Do a bit of research and (don't inject yourself) look up and read about injection techniques and "off-label" techniques so you know what your injector is up to and what to ask for. The informed patient is the happy patient!