The person is a huge variable. How physically fit and overall healthy someone is in the first place will make a large difference. The heavier someone gets the faster they will get dehydrated. That can even apply to people with too big of muscles and not just too much fat.
We should plan for what is known not what if regarding food and water. Regardless of our physical healths we should except and plan around the big 3s and use it as our template for any survival situation, climate plays a role how dehydration affects a person such as hypotonic or hyponatremic dehydration, I have seen hyponatremic dehydration take some of the most healthy and fit and drop them like a rock less then three days in south america.