Battery life span, not to be confused with your battery life, is the length of time your battery has before you need to replace it. The rechargeable battery in your iPhone should work at optimal capacity for (hopefully) a couple of years at the very least, but that depends on a variety of factors, like environmental temperatures, how you charge your battery, drop damage or a faulty battery. ("Battery life," on the other hand, is the amount of time your battery lasts before it needs to be recharged.)
