Although the cable length variable is system-dependent, I've been told numerous times that my 20' speaker cables are too long & are degrading my system. I don't know if that's true, but certainly it could be (never tried shorter lengths because all my gear is in a rack way off to the side of the room - which helps w/ vibration-isolation). But I do know from experience that longer interconnects are a detriment in my rig, and I use quailty high$ cables. Another interesting note: I was auditioning two same-model upgrade power-cords of different lengths; the shorter one sounded better, go figure? Back to the long speaker-cables: network-terminated cables (& even interconnects) are built to inherently compensate for the particular length in use. My MIT does this: I've heard of one other make but don't recall the maker's name? Regarding max. length, MIT is available up to 40' runs. Gotta love this cable; stil can't believe how good my MH750 works for the mid-level price.
