Having heard some lectures at Oxford University about how our endocrine systems and nervous system change depending on the time of day and our exposure to natural light, I refuse to believe any differences in my perception in recorded sound that are more than about 10 seconds apart. Days apart? - I'm more likely to believe in leprechauns and elves.
Burn-in is not something I've read much about in the UK because it seems to be considered to a large extent audiophoolery. Some engineers appreciate that some capacitors out of the box need a few minutes or perhaps an hour or so to stabilise, and if you are concerned about burn-in, just leave a new component running overnight before listening to it.
Over any long period it's purely psychoacoustic. If a manufacturer or retailer says something will take 100+ or 200+ hours to burn in, I'd just move on to something else. If it takes that long it's not fit for purpose or more likely it's a pre-emptive excuse if you're not impressed by the product.
Rubbish. Burn-in definitely exists. And it’s definitely in the hundreds of hours for teflon caps.