Former Reliability Engineer here. Not sure for bulbs, since my experience was automotive, but I imaging it is similar. In general, we tried to determine all t

Former Reliability Engineer here. Not sure for bulbs, since my experience was automotive, but I imaging it is similar. In general, we tried to determine all the factors that lead to failure - vibration, force, impurities, sunlight etc, then you try to accelerate these. So for sunlight, you might raise the intensity of UV etc. There are settings so X hours of acceleration == to X years normal use. These settings are developed in a variety of ways. After you run the test, you apply some stats analysis and a bit of handwaving and proclaim 25 years.