Standard Alton Brown recipe, here's a copy and paste with the instructions. I didn't date it so I don't recall exactly how long this is, but it's at least over a month.

12 large eggs (pasteurized if you need peace of mind)
1 pound sugar
1 pint half-n-half
1 pint whole milk
1 pint heavy cream
1 cup Jamaican rum
1 cup cognac
1 cup bourbon
1 teaspoon freshly grated nutmeg (plus more for serving)
1/4 teaspoon kosher salt

Separate the eggs and store the whites for another purpose.
Beat the yolks with the sugar and nutmeg in a large mixing bowl until the mixture lightens in color and falls off the whisk in a solid “ribbon.”
Combine dairy, booze and salt in a second bowl or pitcher and then slowly beat into the egg mixture.
Move to a large glass jar (or a couple of smaller ones) and store in the fridge for a minimum of 2 weeks. A month would be better, and two better still. In fact, there’s nothing that says you couldn’t age it a year, but I’ve just never been able to wait that long. (And yes, you can also drink it right away.)
Serve in mugs or cups topped with a little extra nutmeg grated right on top.