I think it turns on based on how far you are from the supercharger? When I set off towards Park Royal, for the first 5 minutes or so the pre-conditioning didn't come on, and I was worried that my experiment with 250kW would be a failure, but then it came on. So I assume it calculates the estimated time remaining, and based on temperature it works out when to start the pre-conditioning. Just guessing, though.
Before setting off, and seeing that the pre-conditioning didn't come on immediately, I thought "new bug with 2019.40.51 or they haven't properly enabled Park Royal in their database, so the car doesn't know it needs to pre-condition".
It was a ~30 minute drive, and I estimate the pre-conditioning didn't come on for the first 5 minutes, so I estimated 25 minutes of pre-conditioning.
Edit to add that the car sounds very different when pre-conditioning is running, so you can tell it's on.