The question: How much junk food can you eat in a sitting?
Answer: All of it.
Cheat day for me: Aww man, that is awfully tough. Let's see - Breakfast - 10 pancakes, 10 slices of bacon, 2 L of OJ, and 2 packs of pop tarts. Wendy's triple with cheese combo, biggie size, with Dr Peppe for lunch. Ben Jerry's Half Baked - entire tub. Dinner - Pizza, large w/ pepperoni.
Mind you, I am partially natural - no steroids (yet), just a bunch of cutting drugs. Mind you the best thing I have done for myself is to get rid of the above said cheat day, and hit the cardio.