The lack of food doesn't matter. It's the lack of water that causes death much faster, and I was under the impression that death by dehydration occurred within a week or so. And even before that there are hallucinations and altered mental status as well as physical weakness (not that we'd notice any of those things in Ms. Schiavo's case). Generally 48-72 hours without hydration leaves you in a pretty bad way, and a couple days after that you buy the farm.