[unfasten]

>Comments on their FAQ like "Should last"

They quite clearly state the usage pattern for the 2 year figure. From said FAQ site (https://onlycoin.com/support/faq/)

    Q. How do you figure that a Coin will last 2 years? What constitutes
    normal usage?

    A. Here’s how we modeled typical usage. On a daily basis, we assume
    you’ll toggle which card you want to use and swipe your Coin 10-20 times
    a day. In addition, you may adjust which cards you want to store on your
    Coin a few times a week. Initially when you’re getting to know your Coin
    you will likely be syncing often. Over time we find that most people
    sync much less frequently once his or her cards are on a Coin. If you
    swipe and sync more frequently on a consistent basis, your Coin’s
    battery will drain more quickly.