With Li-Ion batteries, about once every 4 years if used daily. Granted, with the stock Inc battery being low as is...perhaps once every 2 years.
So...as you can see, with a phone odds are you'll get a new one well before you have a problem with the battery ( unless of course you do things to really affect it, such as heavy usage while charging and letting the battery get very hot. Having it plugged into a non-filtered car lighter port for long periods of time, letting it sit for long periods of time at 0%. etc. )