Probably around -40 or so.
Good anecdote, eh not really. It's just really cold and uncomfortable. My most memorable extreme winter weather walk was in late 2010 when we got hit with a blizzard so bad my work actually shut down early and sent us home, but I ran into the issue of having no place to park. I ended up having to park on a ramp on the edge of downtown, but then ran into the issue of getting back to my apartment, since the buses were pulled for the first time in 19 years. So I ended up walking the over a mile distance in a blizzard so bad that visibility was less than a city block and with tons of snow flying in my face feeling kind of like sand due to the wind speeds. Temperature wise it wasn't anywhere near as extreme cold as like right now, but it was probably more uncomfortable. I remember getting home, taking a shower to warm up, and realizing that I almost lost feeling in my feet as I gradually gained that back.
Is the -40 counting wind chill?
Yeah, I've been in -40 pre-windchill temperatures before but definitely not walking a significant distance in them.