One other tidbit - the reduction in noise for a series of identical exposures is proportional to the number of shots (total exposure time). As you double the number of shots ("subs"), you can theoretically halve the noise in the final stacked image. However as you increase to longer and longer total exposure times, the incremental benefit keeps reducing.
All this depends on light pollution, noise characteristics on the camera, imaging temp and other nasty issue too in practice. But hey, if it was easy I think most of us wouldn't have got hooked on astrophotography.
