Thermos flasks are a bit like Hoovers, the brand name has become synonymous with the item. When I am working outside I like my hot drink. This can be tea, coffee or hot chocolate. However, I can't stand tea/coffee/etc that is pre-made and stored in the flask. I want to make it fresh using hot water from the flask.
There are basically two designs of flask. The super cheapo option available from many petrol stations, which have twin walls with some sort of insulator between the walls. These are inevitably less use than chocolate fireguards , the contents usually gets cold in a couple of hours. The more expensive flasks have a vacuum between the two walls and keep stuff hot much longer. However, there is a wide variation in performance between different makes.
I have got a cupboard full of useless flasks. A couple of years ago I discovered the Thermos Ultimate. It's expensive, but does come with a 10 year warranty.
In a very non scientific test I preheated mine by filling it with boiling water for ten minutes, pouring that out and refilling with boiling water. During the day I took the top off a couple of times and poured some of the contents out. I left the remaining contents (about half full) overnight. The next morning it was still too hot to stick my finger in.