After performing some research on this topic over night, I have come to the conclusion that the duration of an exposure is not the primary concern when it comes to noise. There appears to be a direct correlation between ISO speed and noise. The higher the ISO, the greater the noise. While a higher ISO provides greater sensitivity in low light conditions, thus shortening the exposure duration, it is not generally better.
The resolution and noise reduction obtained at ISO's 800 and lower seem to be the best option for DSOs. Many CCD and CMOS image sensors are usually calibrated so that they give the best image quality (greatest S/N ratio) at its lowest possible ISO speed. The trade off to this is sensitivity so the exposure duration would need to be increased to compensate.
For dim DSOs, a longer exposure will still provide a greater signal to noise ratio, than stacking smaller exposures. (The opposite is true for bright objects such as planets). However, the exposure duration must be determined by finding your locations sky fog limit. If you consistently come close to reaching this limit (Say for example 8 minutes for urban locations, 10-15 minutes for rural locations) then you would be maximising the signal over noise. Over the limit, regardless of how many frames you stacked/combine there would continue to be too much noise with little improvement. There are no hard and fast rules for the exposure duration, one must determine these. Obviously the optical system focal ratio can play a significant role.
Jerry Lodriguess has a great article on determining your sky fog limit. The article is film centric, but can easily be adopted to DSLRs.
http://www.astropix.com/HTML/I_ASTROP/I05/I05.HTM