A little longer than to the back of the bend is OK, but I really like mine to extend to somewhere between the barb and the bend. The P&O is a great fly. Not that hard to tie, and a lot of different ways to fish it. I like to fish it in the film or as a dropper off of either a dry or nymph. This is a version I tie. Nothing but orange silk thread for the body, a little hare's ear for the thorax, and a turn or two of natural partridge.

http://www.flyfishsouth.com/upload/stev ... cn0879.jpg