How long should speaker cables be?

macg19: Nordost Blue Heaven

Start with 12 feet.  Cut off one foot at a time until they are just right.

As others have mentioned above, the prevalent thought for many is long interconnects and short speakers cables.  My speaker cables are currently 6' which is more than enough, but my interconnects (straight RCA) are 18' to my reel-to-reel and turntable.  Many will talk about the need for balanced cables if long runs but I can discern no difference in sound or volume whether I use a 3' RCA or the 18' RCA.  I queried AudioQuest's tech people about it one day too and they said that the longer interconnect is the way to go as well, and, that the cables will pick up little to no noise at that distance nor will you have significant signal loss.  I am currently using AQ Evergreen RCA cables and also AQ Rocket 11 speaker cable.  Of course, your mileage may vary so trust your ears.  Here is a PS Audio "Ask Paul" segment about it.
 

Pithy, and directly to the point. 

@larryincmh if single ended works for you, it works, if it does not, it does not. A long single ended cable is often bad, not because it allows you to pick up more noise, because realistically, who has big noise generators near their audio equipment, but because there is higher resistance in the ground connection which could translate into ground loop noise. If you do not hear it, then don't fret about it.

If someone with some basic understanding of electrical properties and audio systems designed your speaker cable, and is not trying to trick you, then a bit of length in your speaker cables is unlikely to make a difference in the sound. If you are running Maggies and your cables have a lot of inductance or you are running Wilsons and you have a lot of resistance, there could be an audible difference between a short and long cable. If your cables are some esoteric but poorly designed cable where the resistance is equivalent to 18awg, then I would be shortening them from 12 to 6 feet personally. They may sound different. It could be worse or better.