How long should speaker cables be?

Keep them under 440 feet! 

Where possible speaker cables should be kept short and interconnects can be as long as it takes but should be balanced if very long.  But the speakers do need to see some capacitance and resistance load.  So where monobloc amps can be placed adjacent to the speakers, I wouldn't go below 4 feet.

OP says he needs less than 6 feet so he might experiment with 4 feet.  May well be little difference can be heard from 6 feet.  In my monobloc set up I use 6 feet of heavy Kimber silver with four terminations at the speaker end.  Sounds good to me.

Long enough to reach the speakers from the amplifier.

With solid state amps (negative output impedance) the less R between the amp and the speaker, the better real world damping factor you will have.