Start with 10', cut down from there if necessary. On my setup, (38F, 11 PSI, lines inside fridge, taps thru fridge door), 5' gave craptons of foam, 10' worked perfectly.
The correct answer is: it depends on your CO2 pressure, which depends on the type of beer you're serving and the temperature you're serving at. I've made a calculator on my website to help you figure it all out. http://www.mikesoltys.com/2012/09/17/determining-proper-hose-length-for-your-kegerator/