One would hope for the life of the two year contract. Otherwise, Apple will have a lot unhappy customers.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
I would say several years easily, I have a iPhone 2G that I use for testing and have had it since a year after official release and havent had to replace the battery yet...
From what I have experianced under normal use it will last you 4 or more years without needing replacing but it is a wise habit to remove from charger when full and dont overcharge because this shortens the batteries life.
Barring any defect in your battery or any abnormal or excessive use (extreme temperatures for example) your battery should last you well past the 2 year contract. There is no definitive answer to how long though...too many factors to consider. You can read about the batteries for the iPhone on the Apple Support website (along with other info that would likely answer any questions you have about the iPhone). Here is a link specifically related to battery life: http://www.apple.com/batteries/iphone.html