Preconditioning always seems to start as soon as I have a route with SuC stop in the satnav even when that's an hour or more away. I've no idea whether it's actively doing anything for all of that time as opposed to just being an acknowledgement that it will try to make sure the battery is up to temperature though.
When we had the V10 UI it was easy to keep your odometer and Wh/mile card in view on the screen ... I recall seeing the Wh/mile increase when the car reported early preconditioning early in the trip so pretty sure it was actually doing its thing. (I suppose it's possible that Tesla has discovered that "spending" that power warming things up earlier leads to an overall better consumption than doing it all in the last 15 miles ... maybe Bjorn has a video on this but I haven't looked.)