So here is the data varying from 15sec to 2:30min @ ISO1600 from Paramatta using a 254mm Newtonian @ 1200mm focal length.
Flickr Link
The imaging consists of Canon 50D with a MPCC and Astronomik CLS LP Clip in filter. As you can see the saturation at exposure greater than 1min. I wish there was a program that would let you calculate the point at which the data being collected clips off and the noise overtakes.
The contributions above leads me to believe that best bet is lowering ISO and capturing 2min + shots and lots of them!
Brendan: I'll PM you regarding my lightbox and how to use the camera histogram.