Typically it’s not the emitters – the LED’s themselves – that fail. If driven correctly, those have lifetimes of tens of thousands of hours. That’s what the manufacturer is advertising on the box. Yes, an individual LED when driven correctly will probably last 20,000 hours. (Usually more, depending on how pedantic you want to get. The 20,000 hour figure often quoted is the point where the emitter drops to 80% of its original light output.)

LED “bulbs,” the type that replace filament bulbs in consumer fixtures, typically fail in their driver hardware. LED’s run off of low voltage DC and in the base of all of those LED conversion bulbs is a power conversion assembly that steps down and rectifies 120v/240c AC to whatever DC voltage the LED array in there expects. These are inevitably made out of whatever the cheapest passives and semiconductor components the manufacturer thinks they can get away with. These don’t last 20,000 hours, especially not in where they’re usually installed.

The main killer for all semiconductor electronics, which includes both LED’s themselves and their driver circuitry, is heat. This is often exacerbated by the fact that LED replacement modules are usually stuck in enclosed light fixtures designed for filament bulbs that have insufficient ventilation to get rid of the waste heat from the components in an LED module. The insides of those enclosed ceiling light fixtures, the ubiquitous “boob light,” gets hot, even with only LED modules installed. Filament bulbs don’t care because they don’t have any electronics in them and how they work is literally by getting so hot the glow. But LED modules in that kind of environment will invariably suffer an early failure.

The best way you can get your LED modules to last longer is to install them in a fixture where they’ll have a lot of air circulation available or at the very least which is not fully enclosed.