I don't. Or rather, I don't believe that it can take as many hours as people often claim, especially with IEMs. I often experience differences in listening depending on daytime, mood, energy levels, head position, and many other factors. Headphone fit also plays a huge role. The same headphone can sound one way in the evening, and sensibly different the morning after. What changed? Me, and me alone. So I believe that physical burn-in can actually happen to a limited extent, but "brain burn-in" is neverending and will make you hear things that have nothing to do with the headphone itself. I think that a week of daily listening is more than enough to make up your mind about a headphone. I don't want to start a flame or something with this. Just my 2 cents.