That will be brand and style dependant.  Consult the datasheets for the vendor of the LEDs that you are interested in.  I tend to use Stanley at lot, and that info is buried, but is still there.  You just have to look real hard.  Other vendors may be the same way.

I know that blue LEDs don't last anywhere near as long as red LEDs.  High brightness LEDs run at about 1/3 maximum current, give the same brightness as a run-of-the-mill LED when operated at a nominal 15 ma, but will last at least twice as long.  You have to play with the curves.

What brightness level at what lifetime are you looking for?

Hi,

Normally you would be sseing a LI ( Luminious intensity)
degradation of about 15 to 30 percent if you operate at Room
temp Burn in at maximum current rating of the product specs over a period of 1000 hours.
If the degradation is more than 30% eyeball judgement could
tell you cleary that the LED has degaraded and this is the
limit where most Manufacturers wouldnt want to go.
To improve the degradation (ie prevent LI loss ), I would recommend that the LED be operated at a low duty cycle 10%
to 20% with high current levels not exceeding the peak forward current of the product spec. Osram Semiconductors may include degradation curves in their product spec.
Joy to you.

It depends on the LED, the way you are driving it, and somtimes (but one hope not) the way you mount it in your product.

If this is a serious concern, press the manufacturer of your device for details and life test data for the part you are planning to use.

Even though red GaAs LEDs have (in general) become pretty stable over the years, some more exotic LEDs, especially the white types, can have pretty dramatic degradation curvers. Again, the only way to answer the question is to work with the manufacturer.