Without more information, it is not possible to determine how long it took for Mr. Jones to have 50 mg or less of the medicine left in his body. Factors such as the type of medicine, the dosage Mr. Jones was taking, and his metabolism can all affect the amount of time it takes for the medicine to be metabolized and eliminated from his body. It is important for Mr. Jones to follow the instructions given to him by his doctor and to consult with his doctor if he has any questions or concerns.