I think it would depend on how faint of details you want to reveal, and how clean you want them to be. With 100% Q.E. and 0e- read noise (and let's also say zero dark current), you have a perfect sensor. However, even with a perfect sensor, you still have to deal with shot noise in the signal itself, as well as shot noise from airglow (if you are at an exceptional dark site). You would need an extremely long exposure, hours, to image IFN with clean results, for example...because it will still have shot noise, and it is extremely faint.
Full well capacity is a function of pixel size (area of the photodiode). So, maybe if you were using 24 micron pixels, and if you had perfect polar alignment without any drift, no wind, no other environmental factors that might ruin the exposure, no frame intrusions like airplanes, meteors, satellites, etc. Then, when you have a perfect sensor and perfect imaging conditions and perfect tracking at a perfect dark site...you might be able to simply expose for hours on end until just before twilight, then end your exposure.
That would really indeed have to be one heck of a PERFECT night, though. I've never experienced such a night myself. Tracking issues and frame intrusions alone would ruin even a single 1-hour sub, assuming I just wanted to keep that sub as my final image. There are key benefits to stacking, such as sigma rejection, and dithering, which allow us to get more ideal results than a single exposure allows. It is more reliable as well...because there is always going to be something that screws up long subs, and if you are going for just one big long sub over an entire night...then if anything messes it up, you just lost your entire night, rather than just "one sub".