I know that xanax (alprazolam) 2 mg make u sleep but for how long?

Depends,: If you have never taken xanax (alprazolam) this could make you sleep for hours. If you use or misuse xanax (alprazolam) you may have developed tolerance and you may not feel drowsy. This a potentially dangerous drug and you should take it only when prescribed and supervised by your md. Be careful, please.

Max only few hrs.: Xanax (alprazolam) is a short acting med that crosses the blood brain barrier quickly & binds to GABA receptors providing instantaneous relief from anxiety & sometimes a soporific effect. The half life is approx 12 hrs; however, the actual sedating effect lasts for a few minutes to 3-4 hrs. You may need higher doses over time to fall asleep. It is better to find healthier ways w/o meds to help insomnia.

2mg: A lot depends upon how how long you used this medication and whether it's already built up in your bloodstream. Most of my patients who take it at bedtime sleep through the night on it.