Like with all lithium ion batteries, it will eventually die.

The length of time that takes depends on the build quality, how much you
use it, how hot it gets when recharging, and heavy a load you place on
it.

There isn't anything uniquely good or bad about the iPhone battery
itself, so your results will vary.

I burned through my first Razr battery in about six months (to the point
where I had only a few hours of standby time), my mom is still going
strong on her first battery after two and a half years.