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Just try it. Put the camera in the dark at room temp and take a dark frame a 5, 10, 15, 20, 30 mins etc. Look at the images. When the dark noise is becoming a significant anount of the image then you won't be able to extract reasonable data from the image no matter how little sky glow you have.
Reducing the temp will let you go longer but the room temp test will be a good baseline.
If you find a 10 min exposure has lots of pixels that are saturated then you need a shorter exposure or you lose too much data.
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