Usually around 3.5", but it always varies depending on your lift. The longer shafts are meant to work in conjunction with a SYE (Slip Yoke Eliminator) which shortens the transfer case length and allows the longer shaft to fix your driveline angle. You can sometimes fix this by using a transfer case drop instead, or having adjustable control arms and changing the pinion angle.
All of this is to try to keep your drive shaft from binding as much and make your u-joints much happier.