My commute is 40-60 min depending on the route I choose, and I pack it in at about -13 C/ 8 F. At that point my feet are in winter shoes with neoprene booties, and starting to feel the cold. I'd need chemical warmers, thermal booties (looking into PI's) or switch to platform pedals and hiking boots to be warm below that. Oslo rarely gets temps that low for more than a few days, though, so it's not worth the hassle.

At that point my bike is usually protesting too, all the lube is pretty stiff and half the gears are "gone"... so I put on a down jacket and go sit on the metro with a book instead