Maximum heating power in my 2015 Tesla Model S 70D is 6 kW. Travelling for 100 km at 100 km/h costs about 25 kWh. I drive in shirtsleeves and barefoot in the Norwegian mountains at -20⁰C and the heater doesn't seem to be running hard. So unless you are traveling in severe arctic conditions the heater really isn't more than a few percent of the load.
Teslabjørn has a video where he turned his Model X into a sauna getting 40⁰C inside while it was -10⁰C outside.
Had a model 3 for 4 years, have another EV now.
It really depends on the type of driving.
Sure, if you are making a long road trip at high speeds then its probably negligible though still noticeable.
But being in Northeast US with constant traffic..
I used to have to park outdoors so the car would get cold soaked down to 20F in winter, and never really have sufficient time to warm up unless I was going for a 1hr+ drive.
Winter driving local roads, below-25mph stop&go, 2-5mi trips running errands..
Could see some really crazy consumption numbers pop up like 500-800Wh/mi+ versus the rated 250Wh/mi. Now it doesn't necessarily amount to much because it's on short single-digit mile trips, but it does happen. This stacks with the general cold weather efficiency losses of EVs..
Teslabjørn has a video where he turned his Model X into a sauna getting 40⁰C inside while it was -10⁰C outside.