Hmm... How long is a piece of string?
A little more information is needed.
Are the lights running through a relay and then on to the battery (fused of course)?
What size cable is being used?
What wattage are the lights?
What amperage is the relay?
I'm assuming the lights are 150W a piece.
In that case, you will require cabling that will support 600W @ 12 volts, so you probably would want something no smaller than 12AWG, and a relay capable of switching between 60 and 80 amps.
For cable / power ratings etc...
http://www.rbeelectronics.com/wtable.htm