Slightly more often than it breaks.
beat answer here lol
The life of a serpentine belt depends on several factors like mileage, load, tension as well as the heat. So you will know if the belt needs replacement if you hear squealing, severe cracking on the underside of the belt, and an angry-sounding alternator.
As mentioned below, look for cracks, chips, and cuts. Also, inspect the inside of the belt (with the ridges) by feeling the rubber; if it feels hard and brittle, it needs replaced. Uneven wear on either side of the belt is a possible sign that the tensioner or another pulley may need replaced as well.
Serpentine belts should last between five and ten years. In my opinion, if it's showing cracks in the outer casing (the flat part), you really ought to consider purchasing a new one and installing it, or having a new belt installed.
. Should tell you in the owner's manual, but the best way to tell is by inspecting the belt. If it has any cracks, chips, cuts, &c - replace it. For more info, try Googling "serpentine belt inspection" (w/o the quotes).