I think the two main limits to regen at temperature of the battery and of course if the battery is already nearly fully charged.* Turning on 'Climate' with the Tesla App will get the stators to start generating heat and this heat goes to the cabin and the battery. The battery heats up slowly, so I often start a half hour a head of time. I can actually follow the battery temperature with 'Scan My Tesla' - I've found that 70-72 degrees is the sweet spot for one of my commutes that starts with a really long steep descent [down from a ski area]. Also, if you've heated up the cabin with energy from "shore power" then you'll waste less energy on your drive. If you want to be real efficiency, get a blanket, gloves and a hat and stay warm with the seat heater - that uses much less energy than heating the cabin [with one passenger]. I suppose four seat heaters might use the same energy as heating the cabin.
*there are other factors though, when I look at the dots showing regen limitation, or 'max charge' on 'Scan My Tesla' there is a slow improvement over a period of time even though battery charge and temperature don't really change. Not sure what this is about.
