Life of iPhone 4 LED flash?
From Wikipedia:

LED Flash (Illumination)


  Although they are not yet at the power levels to replace xenon flash devices in still cameras, LEDs (specifically, high current flash LEDs) have recently been used as flash sources in camera phones. LEDs are expected to approach the power levels of xenon in the near future and may replace built-in xenon flashes in still cameras. The major advantages of LEDs over xenon include low voltage operation, higher efficiency and extreme miniaturization.


So, now that we know that we have a LED (Light Emitting Diode) in there, let’s see how much should that live (according the the super-experts @ wikipedia):

LED Lifetime and Failure (read all that for more info)

Short answer: 


  "Typical lifetimes quoted are 25,000 to 100,000 hours but heat and current settings can extend or shorten this time significantly.”


Now do you think your iPhone will last 25,000 hours (worst case scenario for the LED)? That’s a little bit less than three years. It’s certainly possible for an iPhone to last much more than that (I still use a 1s-gen iPhone), however, I highly doubt that you’re going to use it with the flash ON, for three years, non-stop. :)