LEDs have full intensity instantaneously.

Bulbs burn out for various reasons
1. Surges, spikes - could be due to harmonics, ...
2. Heat
3. Combination of the above.
4. Vibration
5. Statistical outlier (MTBF - does not mean that all bulbs will have the rated lifespan).

Your KD is misinformed. LED's do not need to warm up.

Sherwhy is absolutely correct. LEDs light instantly.

We also had problem with lights burning out after very short time, attributable to a 'bad neutral connection'. An electrician had us send the following in a letter to our electric company. After talking with our neighbors, who had similar complaints, the electric company dug up the street and apparently fixed the 'bad neutral connection'. No more problems!

Letter to electric company:

For some time, we have noticed that:

-- we briefly lose power several times per month
-- we have several power surges per month that cause our electronics to reset
-- our light bulbs burn out after only a few weeks of use

We had an electrician check our panel, meter box, and internal grounds, and everything was fine. He told us the problem is likely the neutral connection, and referred us to YOU, our electric company.

The voltage at our house is slightly over 120v. Sometimes it is 125v. Utility companies do this to push more power into growing areas. Our bulbs seem to go out before they should. I recently installed dimmers. Cranking the light back just slightly should help the bulbs last longer. The dimmer I installed (Lutron Maestro) also ramps up and down the voltage so they don't instantly hit the bulbs with full volatage. We'll see if this helps.