Put cider, and rum if you are using, in a medium sized saucepan over medium heat and gently boil, stirring occasionally, until reduced by half and syrupy in texture. This may take anywhere from 12-25 minutes, depending on the strength of your stoves heating element. If you boil it down too much, remember a couple of things. If you decide you want the cider more thick, once removed from the stove, it will thicken even more. If you have left over syrup you want to use again, simply, and gently, heat it back up and it will thin out. Also, As you reduce the cider, stir more frequently so that it doesn't scorch or burn on the bottom.