Speaker Cable Length - Effect on Sound

What effect will longer speaker cable lengths have on sound? For comparison, what will the sound difference be for a 3 ft cable vs. a 10 ft cable?
One other question, what length should one cap a speaker cable length? Thanks!!
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.
As a general proposition, shorter runs are always preferable to longer runs. Assuming you have a heavy gauge of wire, however, you should not hear any difference between a 3' and a 10' run. However, as you exceed 10' in length, the cable thickness should be at least 12 gauge. While length is important, there are other factors that come into play as cables become longer. Capacitance, resistance, inductance, and impedance all influence the quality of signal transmission. You might try looking at KimberKable's Web site, which contains a table that shows the measurements for various kinds of Kimber products.
I went from 7' to 18" in the same cable, Stealth Ultimate Ribbon. This is high enough gauge for 7', no obvious problem there. But I was brought MUCH closer to the music with the 18" cables. Unfortunately, this short a length calls for monoblock power amps, for stereo listening. If you can plan to have monoblocks in the future, near or more distant, you won't be disappointed by the improvement--if my experience is anything to go by.
I have a variant (as usual) question. If you are using REALLY short cables (like 2 feet with my monoblocks), does the speaker cable quality matter as much ?
I also have 20 foot MITs. They claim their terminator box tweeking compensates for the length. They sound good to me, and a lot better than the 18 foot ones they replaced, so I assume it is true.