How long should speaker cables be?

+1 on everyone who said long enough to reach the speakers! 

from any vintage McIntosh manual. They have a chart with recommended guage/distances.

"Added resistance causes depreciation of damping characteristics, modification of frequency response and reduction in power to the loudspeaker. Use lamp cord or wire with similar insulation to connect the speakers to the amplifier. In all cases, the leads to and from the speaker should be twin parallel conductor or twisted pair. When using 8 ohm speakers and for the normally short distances of under 30 feet between the amplifier and speaker, #18 wire or larger can be used. For distances over 30 feet a larger diameter wire is required. Select the correct size from the chart below. The DC resistance of the speaker leads should be less than 5% of the speaker impedance. Resistance of the leads must be computed for the length of wire both to and from the speaker or speakers.

MAXIMUM WIRE LENGTHS Wire Gauge 18 16 14 12 10 For 4 Ohm Load Feet 15 25 40 60 100 Meters 4.57 7.62 12.19 18.29 30.48 For 8 Ohm Load Feet 30 50 80 120 200 Meters 9.14 15.24 24.38 36.58 60.96"

The world has moved far beyond lamp cord mentioned in those days, you have to decide if you hear a difference, and it so, preferred?

I make my own out of CAT 8, pure copper, great insulation: 8 individually insulated small diameter solid strands add up to 13awg. Ridiculous, but preferred! Hah!

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.