Pour a gallon of apple cider into a large, sturdy stock pot or Dutch oven. If using cast iron make sure it's enamel coated.
Use a wooden skewer, chopstick, dowel, or ruler to measure the depth of the cider in the pot. Dip the skewer into the cider until it hits the bottom of the pot. Using a permanent marker, make where the cider's surface is. Section the rest of the skewer into eight sections. The boiled cider will be done when the surface simmers down to the last marking on the skewer. (You'll notice I used a cardboard straw since I was fresh out of wooden skewers that day.)
Bring the cider to a boil over medium-high heat. Reduce the heat to a simmer and simmer uncovered (no lid). Do not speed up the process by keeping the cider at a rolling boil the entire time. This is a low and slow process, so keep it at a low simmer. Every stove is different, but my perfect setting is low to medium heat.
Skim the foam from the top of the cider as it simmers.
Stir the pot once every hour until the cider simmers close to the skewer's last marking. At that point, stay close and stir every 15 minutes or so. The process could take 4, 5, or even 6 hours. I've made boiled cider multiple times, and it consistently takes 5 hours.
Pour cider into a glass measuring cup. If it hits the 2-cup line, the cider is done. If you have more than 2 cups left, pour it back into the pot and continue to simmer until you only have 2 cups. Don't simmer until it thickens, just until you have 2 cups. If you cook it until it looks like thick syrup or molasses, it'll cool and be too thick/hard to use.
Strain the cider, if desired, then pour it into a jar.