The length of time you should stand on a BOSU ball depends on several factors, such as your fitness level, experience with using the BOSU ball, and the specific exercise you are performing. For beginners, it is generally recommended to start with short periods of time, such as 10-15 seconds, and gradually increase the duration as you become more comfortable and confident. For more advanced users, the duration may be longer, depending on the exercise and your fitness goals. In general, it is recommended to start with short intervals and gradually increase the duration as your fitness and stability improve. It is also important to use proper form and technique, and to seek proper training and instruction before starting to use the BOSU ball.