Well that is no fun. With a unlimited number of questions i could guess what you were thinking as well. Kinda defeats the purpose. 20 is the max i will go.
No the max is 40. He guesses at 20, 30 and 40 questions. Sometimes he just needs more info than the first 20 questions. If he knows it sooner than 20 he'll guess then. Fastest was Will Smith. Guessed him in 6 questions.