Anti wrinkle injections start to take effect usually within a few days, with full effect at two weeks.
Most clients have it repeated every 4 months, when the muscle function has started to return. It is usual to retreat when there is about 50% of the muscle activity back.
The effects will usually wear off completely by six to eight months and there is no evidence that it has any long term effect on the muscles that it was injected into.
However, by relaxing the muscles for period sof time, the inclination to use them is reduced, and this 're-education' effect is also helpful in the prevention of further lines or wrinkles.