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In Summary:
The reality is weird as its per pixel, not per subject,
Say for stars, after a few seconds your usually clipping unless your shooting HDR to recover them, this means longer exposures generally loose signal on bright stars, the longer you shoot, the more you loose, however apart from clusters people usually don't care enough on blowing out bright stars,
For nebula, its faint, but there is a limit where even it can start clipping, past that point those pixels start loosing signal
For both cases, sky brightness starts eating up the dark point of the image, meaning the second you clip, your loosing signal
Getting into other parameters, there is tracking accuracy, seeing, guiding accuracy, atmospheric attenuation, that can work to smear detail over nearby pixels
There are technical issues, e.g. how long between needing to refocus, how long your FOV doesnt have any satellites, planes or birds cross, and what your lowest ISO is
To that end lowering ISO only really makes sense if you gain dynamic range by doing so, you generally aim for the knee where dynamic range starts to drop as that will be where you can capture the highest SNR per unit time,
On the camera there is all kinds of fun non linear noise sources, some that increase with time like shot noise and dark current, and others like dead pixels, not usually enough to lower SNR, but they factor in like the sky brightness chewing away at the black point
All of these in a way chew away at how much contrast you can capture in a single image and put a upper limit on how long you can image for,
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