How long should speaker cables be?

Just who, if not you, is the expert?  Try what makes sense , listen and modify, but first get the room acoustics right and only change one thing at  a time.

If I have to go long, it will be because of bass passed through my second FLOOR listening room and anything that I do to tame vibration issues is far more critical than long speaker, or long balanced interconnects.

I am over 2 meters tall.  When people tell me I have really long legs. I reply: " I don't think so - they are just long enough to reach the floor"

Long enough to reach....oh.

Wrong 'cable'...the male/female threw me....

Interconnects can be as long as you kneed it to be.  Works for me...;) *tic*

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.