The original battery in your iPhone can last a very, very long time. I know people with iPhones that are over five years old and their iPhones STILL have their original batteries. However, the big caveat here is those iPhones no longer last 12-20 hours on a single charge. An iPhone with a five-year-old battery might only be able to go six to eight hours before needing a recharge.
This isn’t Apple’s fault. Though Apple is one of the most technologically advanced companies on the planet, the iPhone maker still has to obey the laws of psychics and chemistry. Apple’s iPhones do use the latest stable battery technology–the lithium ion battery–however, lithium ion batteries still rely on chemical processes that naturally degrade over time.