For just lobbing gobs or chunks into inside the sandbar holes I would say 14 - 20 inches. That is a good compromise for both natural bait presentation, allowing it to move with the wave action and castability.
If you are fishing a washing machine, an area of real busy water like an outsuck on dropping water, I might shorten it to 6 - 12 inches if tangles and twist-ups become a problem.
Of course, the longer you need to cast the shorter it should be. On the Outer Banks thier "Hatteras Rig" is a fishfinder with a 2-3 inch leader. Many guys down there are going even shorter using a "Cannonball Rig" which is no
leader, snell the hook on the shocker with the slide/swivel butting right on the eye.
If I'm using a fishfinder it is Hatteras style, the slide is a 130lb swivel and the leader is 3 -6 inches long
Another route, which is the one I use 95% of the time, is a clip-down pulley rig which keeps the bait from helicoptering in flight but once on the bottom works like a fishfinder. With a long pulley you can have a leader as long as you want and have it cast very, very well.