Peter, see
my thread here for an example of imaging in light polluted versus dark skies.
In my back yard, I can't even see the horse head in my subs with several minute exposures through a telescope... it's just red from light pollution. I need to shoot lots of frames and stack before I can see it. From a dark site, a 1 minute sub clearly shows the horse head.
According to the maths figured out by some smart people (such as
here and
here), 1 minute of shooting at the dark site I went to would be equivalent to about 30 minutes in my back yard. My personal experience so far supports this.
By the way, are you dithering between each of your frames, i.e. moving the camera by small amounts so that the target object is always at slightly different locations on the sensor? If not, the effectiveness of stacking will be greatly reduced.