Barring catastrophe, as long as he wants. And since Drew is a smart guy, I'd guess somewhere around 40 when the tradeoffs between what he wants to do with the rest of his life balance against physical decline. He could probably play longer then that, but I'd guess it'll be easy for him to know when to walk away.
Not sure I understand the 4-5 year predictions. I'd guess he'd have something left in the tank at 34 or 35 barring something unforeseen and again I think Drew will have the luxury of deciding when to hang them up rather then having it forced upon him.