This is actually a really good question. Of course, it's easy to be skeptical. 25 years? Considering 25 years ago is when I graduated from high school, it's difficult to imagine them lasting that long. Then again, I still have a CD player I got as a graduation gift. It still works just fine.
It's important to remember that LEDs are fundamentally a different technology than either incandescent, halogen or fluorescent. LEDs are really mini electronics, much like your cell phone or its charger. LEDs (in most cases) must take your high voltage (110 volt) alternating current (AC) and drive low voltage direct current (DC) LEDs. By stimulating the electrons, it creates light and very efficiently.
So an LED bulb must have something called a "driver". This is a set of electronics, much like your cell phone charger, that must do exactly that. The other way it can get around this is by using an array of LEDs in serial. Most LEDs are really small voltage, in the 1-2 volts range. In other words, not only do you have to convert that AC to DC, but you have to get that high voltage down to 1-2 volts. The more the driver must step down the voltage, the more expensive the electronics are. Newer LEDs get around this by putting the LEDs in an array in serial. Think of it this way: If you put two 1.5 volt batteries in serial, you get 3 volts. Much the same, if you put two 1.5 volt LEDs in serial, you can drive it with 3 volts instead of 1.5. Many LED chips put multiple LEDs in serial on a single chip, and multiple chips add up. Many A-style LED bulbs may have 18 chips with 4 LEDs on a chip, and can be driven with voltage similar to 110 volts. This drives the costs down substantially.
So in short, there's two major parts of an LED bulb, the driver and the LED array. If one of those parts fail, the whole bulb will fail.
That said, electronics for the most part are pretty stable. Your cell phone charger is going to work pretty well, particularly if it's from a reputable company. Your LED bulb is much the same.
If one thing is going to kill those electronics though, it's heat. Many LED bulbs are not rated for working in enclosures because they cannot dissipate the heat. Many of the components are rated to last so many thousands of hours at a certain temperature. The higher the temperature, the less time they're rated to last. So check to make sure your bulb is rated to work in an enclosure before you put it in one. (Only bulbs that are rated to work in enclosures can earn the Energy Star label, so that's a good thing to check for.)
Barring the electronics failing, LEDs, like most other lighting technologies, will get dimmer over time, about 1% or less for each 1,000 hours of use.
So that's where the 25,000 hour rating comes from. The electronics are rated to last for 25,000 hours at temperatures the bulb is rated to operate at. The LED array will get dimmer, but instead of failing "catastrophically" (i.e., burning out), LEDs are rated to be past their lifetime when they reach only 70% of their initial brightness. They will still work, but they won't be as bright.
The process of replacing bulbs that are too dim for most use is called "re-lamping". It's common in replacing fluorescent bulbs. Many LED street lights are expected to last for 7-15 years. They are expected to reach their 70% limit in 7-10 years, but can continue to be used for many years after that. For your use, if a bulb gets noticeably dimmer (let's face it, 10-15 years from now), you may wish to replace it, but use the old bulb in a more marginal place, such as a closet where you want instant on but don't need the light to be as consistent.
LEDs also are subject to very fast changing technology. CFLs in particular are a very stable technology. They're about as cheap and efficient as they're going to be. CFL technology such as dimming has been largely replaced by LEDs, as they're similar in price but performance in LEDs in much better and LEDs last longer. The two largest areas of improvement are price and efficiency. Early 60 watt replacement bulbs were $30-40. They now can be had for as low as $8, even without subsidies from power companies. Except them to drop even lower. In efficiency, the early LED bulbs were about 12-13 watts. Now they're as low as 9.5 and dropping. For that reason, I'm telling people not to replace all their bulbs (particularly their CFLs) with LEDs right away. I wait until CFLs burn out, then replace the LEDs I use most with the newer, most efficient bulbs, and then use the older LED bulbs to replace the CFLs that burn out. I have one bulb in my living room that is used more than any other light that has gone from 30 watts, to 26, to 23, to 18. It doesn't sound like a lot, but at 5 hours a day, that difference does save me about $3/year.
So like most electronics, LED lights will probably either last a couple months or a decade or so. Also like most electronics, keep your receipt for the warranty period (3-5 years for most LED lights, I keep my receipts in the same place I keep my extra light bulbs). If it fails before then, get a replacement. If not, you're probably going to enjoy it for a decade or two to come.
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