After 15 minutes Flowpillow will turn off by itself. This is the recommended duration for one muscle group. In other words you can with benefit for example massage your lower back for 15 minutes and then move on to another 15 minutes somewhere else on your body!
The timer has been set to 15 minutes to work as a reminder to not massage one specific area for too long. Since muscle fibers also break down during intensive massages and can be broken down too much, similar to during exercise.