In a large, heavy stockpot (such as a Le Creuset dutch oven), bring cider to a boil over high heat. Reduce to medium-low heat and cook uncovered for 4-5 hours, stirring occasionally.
The boiled cider is done when it can coat the back of a spoon and the consistency is like maple syrup.
Transfer to a clean jar, cool, then use immediately or seal it tightly and store in the fridge indefinitely.