Diesels really aren't as bad to start as people make them seem, especially glow plug Diesels like the Duramax. It's probably going to get down to at most 0-10 F before you start running into real issues. I know in 50-70 F they fire right up with literally an instant glow cycle. I'd worry about the fuel gelling before I'd worry about it not starting due to cold weather. It is, however, better if the block heater is plugged in. It may also be smokey and rough in cold weather until it warms up a bit.