I think that the duration of anxiolytic effect will differ greatly amongst different people depending on tolerance, dosage, and just how well the drugs works for you in that respect. The half life is often stated as it is a more concrete value than level of easement of anxiety. There is also both IR (instant release) and ER (extended release) preperations of alprazolam available.
This is a topic you should discuss with your doctor and it would be helpful to mention to us if you are planning on taking it as daily maintenance or just when needed. Remember that benzo's are rarely a good long term solution.