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Forget 5 minute subs, even with winter weather the internal heat built up by the camera processor, sensor and battery (yes it gets hot too) will give you a mess of noise.
I agree two minutes is about all you can manage before the noise threshold gets too high, and really a long cool down pause would help as well, otherwise you build heat on heat and the images get worse and worse. This is why we use cold finger cooled dslrs, so the sensor can be held at 0C or lower. A battery eliminator adaptor also helps.
You can test yourself, just find a way to read the EXIF header temperature info on each image file, this will show you the processor temperature (as no dslr actually has a sensor temp probe), and you can watch it climb as you shoot a sequence of long subs.
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