I found this product most helpful for soothing pain from arthritis in my lower back. The heat lasts at least 12 hours, just as it claims. One has to be very careful about the intense heat the pack generates and make sure that there are at least three layers of cloth in-between it and the skin. Otherwise, you could get a burn.
We used these camping when the nights were in the 30's. Toss a couple in your sleeping bag and get a good night's sleep. They take about an hour to reach full heat, so open ahead of time, and get into a warm sleeping bag. They are not as hot as a similar product on market, but there aren't the skin burn from direct contact concerns the others had. We even tucked them inside long johns to get warmed up after a big chill. They really do last 12 hours with hours 2-11 being the warmest