You can use it whenever you want. For as long as you want. At a certain point, it fatigues the muscles (wears them out) and they are tired and RELAXED. That might be the goal. So its an individual thing. I've had three tens units over 30 years. They are great for many uses because I have muscles that go into spasm - ALOT. I've had 3 MAJOR auto accidents and work repetitive overuse problems. Just last week I started a new rehab for post-op in my neck, plus a concussion from a fall giving me vertigo (as I said above). They put the unit on my neck, making the vertigo go away for 4 days. I'll be doing it all over again today! Its NON MEDICINAL and there is zero downside... so for $30 you can't go wrong. If you don't like it, you can send it back! I use mine for about 30 minutes a sitting... or if I am watching tv maybe longer. Any time of day!