I like to keep mine as short as possible. Just enough to slide the caliper off and hang it out of the way, for service or whatever. More than a few guys have gotten lines too long, and ended up with a mushy pedal.
Braided brake lines should only be used from the front frame to the calipers on each side and from the frame to the rear axle on typical applications. Keep the braided brake lines as short as possible while offering flexibility between these components. Long braided steel brake lines are not conducive to optimum brake performance . . . .