Stereo, it sounds as though you're not familiar with Nordost's line. It is in nearly every review of them that their break-in time is extensive. It has rarely been my experience that a cable that doesn't sound good when you insert it in to the system does not improve over time. I say this from personal experience, and also professional (working at one of the high end magazines).
Akg's experience most closely mirrors mine, so thank you Akg. And my experience would probably also mirror Xti's, except I've never had a dealer break them in for me. Besides which, I have a tremendous sense of curiosity, and I actually LIKE listening as a component goes thru its changes. It also makes it easier to respond thru empirical experience, rather than having to guess. When I was newer to audio, I don't think I noticed break-in as much as I do now, but then, I didn't know to even listen for break-in. I would have been oblivious to finer nuances in sound that I now routinely listen for. If I cannot hear a semi-quaver on a flute, for example, that component - whatever it is - is not for me. I need the "human touch" in my sound reproduction, so it should not be the sound of a mere instrument, but the sound of a human being PLAYING an instrument. Those are two completely different methods of reproduction. One sounds "real," and the other sounds "reproduced."
Nordost still takes more time to break in than is usually acknowledged, but so does Shunyata's top tiers of cables. I should know, since I own both Shunyata and Nordost. Transparent didn't used to take quite as long to sound "right," but Nordost? Weeks. And that's playing it 24/7.