At least 2 hours, IMO. 4 is preferable, and I've held 2 briskets in a picnic cooler for 6 hours and they were still too hot to handle.
If you're concerned with the cooler temperature dropping, you can warm it up a bit with warm to hot water. Be careful, it is easy to melt thermoplastic liners. You can also heat up a couple of bricks or other heavy objects, wrap them in towels and stash them in the bottom as a heat reserve.
Probably the single biggest thing will be to put the briskets in the cooler, and walk away. Don't open it until service. Every time you open it, you'll lose heat.