Does xanax (alprazolam) start working immediately? or is it like an anti-depressant where it takes a couple weeks?

Quickly: Xanax (alprazolam) and most benzodiazepines typically take 25-45 minutes to take effect. They usually last between 6-18 hours. They do not need to "build up" in your system which makes them easier to use on an "as needed" basis. That's also what can make them habit-forming as well. Be careful and use them with the respect they deserve. Trust me!