Northwave winter boots (mine are an older version, but you can see
the current model here), with BBB water/windproof overshoes, plus
Lenz socks that have an integrated battery-powered heated sole.
With this setup, I've done 6+ hour rides in temps of 20-30 F (-5 to 0 C). I'm sure that other people are tougher than me and ride in those temps with far more minimal footwear, but I like being comfortable and my feet get chilly quite easily. With my current setup my rides are not limited by temperature because it very rarely gets colder than that here; road conditions at those temps can keep me indoors though, so maybe I need to get a trike with studded tires next
