SASless' response is basically what i'd say. i fly the 350 doing seismic exploration year round and we are limited by what the RFM states which is -40C. there are other considerations of course, such as if you are taking people out with you then maybe you limit should not be so extreme just in case you have mechanical problems due to the cold and the poor saps have to stay out over night. this is not so much a concern if the ground crews have road access.
our machines also stay out over night, covered, and with electrical heaters on the FCU's, oil reservoir, and in the exhaust. this is generally sufficient but with extreme cold this plus a little help from herman nelson in the morning to really get some heat onto critical components including the T/R gearbox is necessary.
i just wish that eurocopter/bell etc etc would put a provision for heated hand grips on the cyclic and collective. if skidoo can do it on their snow machines, then i cannot see why you can't have heated grips in a helicopter
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