The effect takes 4-6 months to wear off due to the slow breakdown of Botox. After repeating treatment the muscle eventually looses its natural strength.
Botox is applied most commonly to the forehead and outer corners of the eyes, because it is here that it affects facial mimics the least. It can of course be applied to other areas of the face as well (e.g. to raise the corner of the mouth or even to prevent gums from showing while smiling).