JohnK is right. Timing is not the issue. And for general home use, length never becomes an issue.
But to answer your question, for 12 gauge wire in use with 4 Ohm speakers like M80s, the maximum length should be no more than about 460 feet.
A 460 foot run results in a total cable length of 920 feet and yields a cable resistance of 1.7 Ohms. This will reduce the power to the speaker by 50% resulting in a 3dB SPL loss. 3dB is considered the minimum detectable SPL change for most people.
For 14, 16, 18, 20, 22 & 24 gauge, the maximum runs are limited to 290, 182, 115, 72, 45 & 28 feet respectively for a 3dB loss. For wires buried in the wall, use larger than 22 gauge to prevent over-heating. Also, small wire like this can get damaged easily during installation and that's why many prefer the larger gauges.
I personally would want no more than a half dB loss. These lengths would have to be decreased significantly (or wire size increased) to meet the half dB requirement.
