The best I've heard from a battery model is 6 hours on medium heat.
As for a vest, that's how I started, with one for my wife. But nowadays, I can't suggest strongly enough to get a full jacket liner. I have one myself, and I'll never go back to just a vest. I wear a t-shirt, the liner, and a Darien jacket (which is just a shell), and I'm good down to the teens. Plugged into the bike, of course.
You will not regret the money you spend on this, I promise you.
Life is not measured by the number of breaths we take,
but by those moments that take our breath away.