When instruments, preamps or whatever are completed they are powered on and checked to see if they work (most do). Then we let them run for 100 hours. Battery operated units such as our microphone and phono preamps are run from a mains power supply instead of from the batteries. After the 100 hour burn-in, the item is tested for proper operation. If it passes, it gets a "big green stick-on dot" on the bottom of the enclosure. Battery powered preamps get their batteries charged for about 20 hours and then they are given a battery life test to make sure the batteries have the proper charge capacity (some don't). If they pass this test they get a "small green dot" on the enclosure bottom.
Why a 100 hour burn-in? For solid-state equipment this is probably longer than is needed. The idea is to find components with early failure defects -- so called "infant mortality." Many years ago the military decided, on the basis of a lot of testing, that 100 hours was appropriate for the vacuum tube equipment of that era. Components have become much more reliable so perhaps 20 or 30 hours would be enough. However, 100 hours doesn't hurt.
The longer an instruments runs without failure, the longer it should continue to run without failure. However, we need to keep in mind that the failure rate never goes to zero . This is important, the rate never goes to zero. Recently we had an opamp fail in the Audio Control Center we use here. It had an accumulated use time of about 2000 hours so it should not have failed, but it did. That's why most every manufacturer has a warranty policy.
