Haven't really got into it, but some folks, like Mapleshade, claim that 8 foot is the minimum length. It may be true of their wire, and the opposite would be true for other brands, I haven't played with it enough. But, a lot of the things Mapleshade espouses, I am in agreement with.
I have used 5 foot lengths and 8 foot lengths and 10 foot lengths, and they sounded very different, but they were different cables. The 5 (AQ Dragon) and 10 foot (AQ Midnight) are from the same manufacturer and same AWG/construction, but one is silver, the other is copper. They sounded night and day different, with my preference being the Dragon so long as it did not make the sound too relentless(which it did for me at the time, but my system is much more relaxed now).