I fell I have to answer myself my questions.
When my mother used to take my temperature (fairly often as she was very protective) about fifty years ago, it was three minutes.
As a teenager, I did a lot of testing on myself and concluded that one minute gives a general idea of your temperature, but three minutes can add one or two tenths of a degree.
Nowadays, small electronic devices beep, some of them after 10 seconds. LOL
My girlfriend told me she had shared my interest for rectal temperature since she was a child and that, living alone, she spent quite a lot of time taking her temperature gain and agai. She says that, at least on one occasion, she kept a thermometer inside her for more than one hour, and enjoyed it.
Now, we take our rectal temperature fairly otten when we make love, as a preliminary or after a sex session we have enjoyed. Then, my gf insists we keep the thermometer in for at least five minutes. I have no objection to that.