Joe Meirhaeghe
Rob is correct. I turn with Steve in his studio on a regular basis. I have personally hollowed several pieces 31" deep with no problems using his ( Advanced Lathe Tools LLC ) 1 1/4" dia. boring bars. We do use the Rolly Munro cutter in it which is easy to adjust the depth of cut on. We do take lighter cuts at those depths.
We also do use a steady rest every piece we hollow regardless of the size of the piece.