Once they are heated, they definitely last 8-9 hours, I used them on a trip to Tromso in very cold weather and they helped keep my extremities from freezing

I use them when it's very cold and I have to stand outside still. Since I put them on, my toes haven't frozen again. It maintains a steady temperature and lasts for several hours (7+).
I would say it keeps a temperature of about 40 degrees. Essentially, it does what it promises