For best results, do not exceed 10% of the load impedance. So if the speakers are 8Ω, then choose a speaker cable that's around 0.8Ω or less. Speaker wire that's 100 feet long would need to be 16 gauge or larger to meet this goal.
For acceptable results, you can double the cable resistance to 20%. That increases insertion loss to 17% which is approximately 1dB.