It depends on what bottomline you use and whether or not you have a rail. I have a 15" long hose on one Phantom with a Hybrid reverse rail and a CP on/off. However, it is about 1" longer than I would like. I think 13" or 13-1/2" will adequately clear the trigger guard without being too long.
EDIT: I am using quick-disconnects at the valve, this will add about 1" to your total length.