I’ve never tried it, not being a drinker of hot water drinks, but vacuum insulation works both ways, so I don’t see why it wouldn’t keep it hot for at least 12 hours. The temperature differential is much greater for hot water to ambient vs cold to ambient, so the rate of temperature change for the hot water is no doubt greater.
I wonder what Kleen kanteen’s definition of ‘hot’ is, to support the several days claim.
