I worked many years in many different types of commercial kitchens. I learned early on not to ask this type of question, because the answer I would without fail receive would be "until it is done." A better question to ask is "How do I know the egg is done?" First, the white changes from clear to opaque (oh-PAKE means you can't see through it). I would say that as soon as all of the white changed from clear to opaque, the egg is done. Leave the egg in the water a few more seconds to make sure the yolk is hot all the way through but hasn't started to get too hard. You don't want to overcook any style egg, nor do you want to cook it at too high a temperature. Too much heat can turn any egg rubbery. Also, eggs poach better if you add a little vinegar to the water before you crack the eggs into it.
