It can depend of many factors. Everyone heals at a different rate. In the beginning if someone is in a lot of discomfort and in really tough shape it may only hold for a few hours. Eventually a healthy individual could hold an adjustment anywhere from a week to six weeks. Things you can do to help it hold longer is to follow the instructions of your chiropractor. Such as any icing, foam rolling or strengthening exercises they may give you. Those will help the adjustment last longer.