Shiraz made an excellent spreadsheet to calculate the optimal exposure time to smother read noise for a bunch of cameras such as the asi071. For my outer Sydney suburban skies, unity gain with an 8"F5 newt it's about 90-120sec. I vaguely remember that it gives me sky values of about 10-20xRN^2; following the calculations mentioned below. If you make exposure time longer there's no appreciable increase in SNR, it just makes saturation of stars worse.
Here's the link to his thread and another about it:
http://www.iceinspace.com.au/forum/s...d.php?t=151000
hhttp://www.iceinspace.com.au/forum/showthread.php?t=160444
Edit: made a mistake it gives 5-6xRN^2 at 90-120sec subs