TENS is a kind of therapeutic treatment, rather than a way to enhance workouts (although there are those who have adapted it for that purpose). Therefore how long you experience benefits from a particular treatment will depend almost entirely on how aggressively you apply the TENS to the affected area and, of course, the nature of the affliction you are treating. That said, instances where TENS equipment actually resulted in damage from “overuse” are rare. So if you need to use the device several times a day there should be nothing stopping you from doing so.