Xanax is a "downer" so even in small doses & no matter your size or tolerance it WILL slow down your respiratory system. If it takes 4-6 hours for a single Xanax to dissolve from your system that means (if abused) your brain could not be getting the adequate amount of the oxogen it needs for up to 6 hours! If oxygen is inadequate for 4 or more minutes a permanent cerebral hypoxia (hypoxia: partial lack), also called hypoxia-anoxic injury (HAI) can occur. This is a serious injury that causes cognitive problems & disability. If you are considering eating a lethal amount of Xanax please take this into consideration as well: Intentional or accidental overdoses do not always end in death, death can be prevented by forced ingestion of Activated Carbon (charcoal), thus violently inducing vomiting. It may save someones life but that doesn't mean it saved their brain. Don't turn yourself into a vegetable!
This varies from one person to another. If someone happens to have been taking the medication for a long period of time, they will have a tolerance. This is done so that they could be able to take an amount that could easily kill someone else. This same principle applies pretty much for any kind of medication that people abuse.