Difficult answer. Xanax, or generic name alprazolam, is used for the treatment of generalized anxiety, panic attacks, and insomnia. It comes in 0.25, 0.5, 1, and 2mg strengths. Maximum recommended dosage is 8 mg a day. Or 2mg 4 times daily. Some people use this drug intermittently, at relatively low dosages, to deal with especially stressful situations, some take it daily at higher dosages. Depends on you and dose.
A week or two. Minor symptoms, such as increased anxiety, insomnia, muscle twitching and others can occur with as little as a week of regular xanax (alprazolam) use, but depends on the dose, and your biology. For severe withdrawal - seizures, delirium, very high blood pressure - at least 2-3 weeks of regular use is needed. If you have been on for some time, it should always be tapered and not abruptly stopped.