It doesn't take long. Maybe ten seconds. Then I go on to treat the person as I would anybody else upon first meeting them, assuming they don't punch me in the face first: politely.
The "None of My Goddamn Business" principle governs my entire philosophy of social interaction. I've found it futile to try to avoid passing judgment at all. However, I don't tend to question the choices of my acquaintances, friends and family, even if I think they're destructive or that they're going to end in tears and regret. I won't necessarily be openly supportive, either, but as long as the effects of those choices have no effect on me, I strive not to let my estimation of them affect our relationship. After years of pondering, I've come to the conclusion that your decision to eat a burger and fries instead of a salad has virtually no impact on the quality of my life.
The moral of the story is, I don't care if you're fat and neither should you, as long as you're not trying to take the food off my plate before I'm finished eating, because that steak is mine, bitch.