According to Better Homes and Gardens, a traditional jelly made with regular powdered fruit pectin takes one to two weeks to set. If opened too early, the jelly will be thin and runny.
In order for jelly to set properly, it needs correct proportions of pectin, sugar and acidity. In addition, the jelly must be hard boiled for about one minute. If jelly turns out runny, either the proportions were off or it was not heated enough. During the setting process, the fruit sometimes separates from the liquid. To correct this, flip each jar of jelly over and set it on its other end once or twice each day. Once the fruit and liquid appear well-mixed, it is not necessary to continue flipping the jars.