For walking, when it’s below -10*C, or so, depending on the wind and how long you’ll be out for. For mooching around camp, I’d definitely need them on before then though!At which temperature do you put on longjohns if hiking and wildcamping?
Let's assume that you wear over it only the usual polyester - cotton mix trekking trousers or the usual army field trousers.

Oh, that really depends on the temperature and on the weather. In subzero temperatures, I prefer overtrousers (insulated or non-insulated depending on the situation), but might just wear baselayer bottoms underneath anyway.
That's pretty much the same reason I knit so many shawls and warm socks.LOL, we live in a 17 Century cruck barn; unless we want the heating running all day, we put thermals on quite frequently in the house
I reckon, if the UK Government dished out two pairs of decent thermals to everyone over 50, we could reduce our 'home heating' carbon footprint dramatically!