When cutting back lilies after bloom, cut back by no more than half their height in order for the bulb to continue to be fed. When the stem turns brown and dries up, then it may be cut all the way back.
Not that I usually manage to deadhead anything....but I have a "surprise" lily this year (I think I must have bought it at an end-of-the-season sale and it didn't do anything last year. Light yellow and spicy-fragrant and rather spectacular! I want to be sure it comes back. In fact, I like it so much I bought a couple of other leftover lilies from a sale table to surprise me next year!
You are welcome, Janet. 
