I have found longer subs to be better. I could show you an equation that supports this, but hey, it simply comes down to do everything you can to maximise signal (e.g more aperture, higher QE) , and everything you can to minimise noise (low noise camera, many subs, deep cooling, ultra clean optics). Is sky glow noise? Not really...just unwanted signal.
With my RC16 I typically grab 15 minute subs from my seriously bright skies...and if using NB filters 20-30 minutes.
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