This would depend on the TYPE of (electro-magnetic )ballast - quoting from the Philips OEM Lamps and Control Gear catalogue:... what is the maximum length of cable i can use from my control gear to my mh fitting. i bought 3x 150watt fittings...
My control gear clearly states up to 10m and it is magnetic.
Like most things in life, there are tolerances on the ballast, cable, etc, and the manufacturers specify their products to work properly at minimum spec, so one can stretch the envelope somewhat... I too ran 150W MH's with ~1.4m cables (but I did use rather thick wires.my cable is +-1.2 metres long