Depends on the pizza. The Pedro's down the street for me, I regularly ploughed through a large, deep dish Meat Lovers pizza every Saturday (don't look at me, it's my one bad meal/non-work out day cheat meal) in less than 15 minutes, sometimes less than 10. Plus the garlic bread.
But my current favourite, Pimpala Pizza, I struggle to get through 5 slices (and only a few bits of garlic bread).