depends how you lay them, I did one of my friends 5th gen with 16ft cables, and I think we could have gotten away with 12ft, I use 12ft in my accord but I run them down the center(also the cables I used were $110 for 12ft and $150 for 16ft so I had a vested interested, especially since I needed two pair).

Easiest say, go buy a soft tape measure for $5 from a hardware store, lay the tape how you plan on running the wires and measure, or use string as was said.

The SNR of cables won't be that serios unless you get very high wality cables, most of the cables I see folks use makes me crings. No shielding, not twisted, nothing.. YIKES!