Stand back... I'm going to use mathematics
Ignoring read noise, dark noise, and the noise from sky glow the SNR for a stack of images is:
SNR = SQRT(N*F*t) where N is the number of subs, F is the target object flux and t is the time for an individual exposure.
You can find this equation in the Handbook of Astronomical Image Processing, here:
http://www.hiddenloft.com/notes/SubExposures.pdf and various other places. It is a consequence of the fact that individual photons arrive as independent random events hence following a Poisson distribution.
If N = 1 and t = 20 mins (a single exposure of 20 minutes) then you get the same SNR as N = 20 and t = 1 min (20 stacked exposures of 1 minute).
There is no magic to stacking compared to a single long exposure of the same total time. The shot noise depends only on the total flux collected.
In the real world where all sorts of noise exist there is one good reason to do as few subs as possible - it minimizes read noise which is incurred every time you read a frame from the sensor.
There are also good reasons to do many shorter subs - it is difficult to guide accurately for very long exposures, it is better if a cloud or other external event only ruins one of many short subs, sensors have finite well depth and will saturate if the exposure is too long, etc.
So, in practice we balance these factors and do subs of a practical length, say 5 or 10 minutes. Maybe shorter for sensors with shallow well depth and/or fast scopes and longer for sensors with deep wells and/or slow scopes. With narrowband filters the flux is lower (read noise becomes a bigger problem) and sky fog is reduced (so the sensor doesn't saturate so quickly) so longer exposures are typical, perhaps 15 to 30 minutes.
If you're so inclined there are calculations you can do to determine an "optimal" exposure time if you know your camera characteristics and sky glow flux:
http://www.cloudynights.com/item.php?item_id=1622
http://www.hiddenloft.com/notes/SubExposures.pdf
http://starizona.com/acb/ccd/calc_ideal.aspx
Hope this helps somebody and dispels some of the common misconceptions.
Cheers,
Rick.