Max length of extension cord for a 5kW load on 240?
You can use a resource like this page to find the resistance (ohms) per meter for wire of any particular diameter:
http://chemandy.com/calculators/round-wire-resistance-calculator.htm

By knowing the current (whatever you are powering?) and the total resistance of the wire (in both directions) then you can use Ohm's Law to calculate the voltage-drop you can expect.  I have a handy Ohm's Law calculator online here: http://www.rcrowley.com/eirp.htm

How much voltage-drop your loads can tollerate depends on what they are, how you are using them, and for how long. Remember that if you overload the extension cords, they will overheat and become dangerous.