With a pre-heated cooler and all the other space taken up with towels, etc, you can keep it hot for an amazingly long time. When I've needed to go for a very long time, I've even heated a couple bricks (an hour or two in a 300 degree oven), wrapped them in towels and used them as an additional "heat source" at the bottom of my cooler. Once they have been heated, the bricks retain their heat exceptionally well.
Bill