Thanks to Klaus for leading me to this paper by Jose Torres where I learned how to calculate Surface Brightness given the size of the DSO and its magnitude. Now, I am wondering if I can use Surface Brightness to estimate the exposure needed to capture it according to the following formula:

m1 - mref = -2.5 * LOG (I1 / Iref)

rearranging terms yields:

I1 / Iref = 10 ** ((mref - m1) / 2.5)

In order to put this into practice I need a reference object and exposure. Let's say that I recently captured M57 with a 30-second exposure and that I am happy with the density of nebulosity on each subframe. Now I want to capture M27.

The surface brightness of M57 is 19.6.
The surface brightness of M27 is 20.2.

According to the above formula:
I1 / Iref = 10 ** ((19.6 - 20.2) / 2.5) = 0.57544

In other words it says that M27 is 57.544% of the intensity of M57.
That makes sense when I compare a 30-second exposure of M27 to a 30-second exposure of M57. The Dumbbell Nebula appears to have half the density on the subframe.

Then, can I safely estimate that I would need an exposure of 52 seconds in order to achieve the same density?
(i.e. 30 sec / 0.57544 = 52.134 sec).

If yes, can I safely use my reference exposure of M57 (a planetary nebula) as a basis for estimating the exposure time for other types of DSO's? Or should I endeavor to establish a different reference for each DSO type and only make intra-object-type exposure estimates?

Thanks to everyone for their help.

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