Housemate and I have settled in to a pattern of having the heat on from roughly 5pm to 10:30pm, and off the rest of the time. Depending what we’re doing in the evenings, the thermostat during “heat” time gets set somewhere between 60 and 65F (16-18C). Two of the seven radiators (my bedroom, and the front entryway) are shut off altogether, and two other rooms (the guest room, and the smaller bathroom) have no radiators and are closed off when not in use– so those all those areas are colder.
This has provided a good opportunity to watch how the house cools. Last night when I went to bed, the kitchen/bunker was at 60F. When the heat turned back on 18.5 hours later, the temp was down to 52. Meanwhile, outdoors overnight it went down to about 20F, and today up to about 32F, for an average of about 26F. It was pretty windy out, too– at least 10mph– which would bring the average wind chill / effective temp to somewhere around 15F.
So, with an average inside:outside temperature differential of 30º (or 41º, if you consider wind chill), the bunker cooled at an average rate of 0.43º/hour.
If the outside temp remained a steady 26º, and the bunker cooled off in a linear fashion, it would take over two and a half days to reach the freezing point inside. But in fact the cooling is non-linear; the colder the house gets, the more slowly it cools further. So I’m guessing it would take at least four days.