for my 8 pin PCIe, 6 pin PCIe, 8 pin CPU, and 24 Pin Mobo connectors i used close to 30 meters. i ordered 20 meters (65 feet) from Nils (MDPC) and that barley covered the 8 pin PCIE connector, and 20 of the 24 pin connector. i think i read somewhere, after i ran out, you need about 15 meters to do a 24 pin connector. of course all this depends on your power supply, i used a Corsair HX850 and the cables are pretty long. the easiest way to get the right amount is to measure the length of your cables and multiply that by the number of wires you have and then multiply that by 1.15, to give you an extra 15%
example
cables measure 61cm, you have a 24 pin, 8 pin CPU, 2x6pin PCIe's total wires equals 44 wires multiply that by 61cm.
that equals 2684cm = 26.84m
then multiply by 1.15
so 26.84*1.15
, and that gives 30.86m