If the drainfield is draining normally, the water level in the distribution box should be at the bottom of the exit laterals. The holes in the lateratls are generally at about 4 and 8 o'clock. This means that there will be about half an inch to an inch of water always in the laterals. Because there is not a signigicant slope on the laterals, there should always be some water in them. The depth of the water in the distribution box will depend on its construction. If the exit laterals are only about 1-2 inches above the bottom of the box, then this is the normal level for your distribution box.