We do usually wait two weeks to reevaluate botox as it can take that long to peak but if one has normally had a result of some muscle relaxation in the first week with a number of prior Botox treatments and on one occasion there is no change at 8 days, there might be some reason for a lack of effectiveness such as a different provider with a different technique, different botulinum toxin, different number of units, different placement, and a different concentration of botox, or an old botox. In my practice Botox is done so often that rarely is botox used that was prepared any more than one working day previously. In some offices botox may be kept for many days in between use and after several weeks the potency may be affected.The information provided in Dr. Shelton's answer is for educational purposes only and is not intended to constitute medical advice. The information provided should not be relied upon as a substitute for consultations with a qualified health professional who may be familiar with your individual medical needs.