Sorry was out doing my shops.
I only use when its going to be below freezing at night. My tank is not insulated as I use it like a giant radiator to keep all my pipes from freezing. Including mine

It's a 3kw heater, I turn it on last thing before going to bed and then off at about 6am, so I suppose that's about 7-8 hours. The water is about 35-40degs C (very roughly) by then, and the whole van nice and warm
The heater does have a thermal cut out set at 60degs C so there's no danger of it getting boiling.
It cools reasonably quickly, but the hose is lovely and pliable and is warm enough to melt thru the snow even in the afternoon. I insulated my old van with 50mm polystyrene sheets and that kept it toasty.