I use one outdoors but am not sure exactly how long it stays hot for outside because it's always cold by morning :lol: but I know that they keep their heat all night indoors. I put one in Alfie and Bubbles' hutch at around 10pm, so I figure that it will at least keep them warm for the coldest hours overnight.
Mind you, when I put it in, they both run off & sit downstairs so I'm not sure they really want it yet. :lol: