I've set a lower limit of 20F for commuting, but it was colder than that this morning and I still rode. (Apparently the thermometer on my deck reads a bit high.) Very cold toes, but otherwise it was ok, so I guess I have to lower my limit.

My limit for recreational riding is about 40F.

(As an aside, I got a bunch of "Hotties" toe warmers for Christmas and they suck. By the time I got to work they were ice cold (hence the very cold toes above). The "Heat Treat" brand from REI seems to last a lot longer.)