I’ve never used anything like

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James Screech

I’ve never used anything like 10 minute exposures, the longest I’ve used is 1 min, most of the time I use 10-20sec. 10sec will get me down to around mag 11 with errors about 0.03. I would only accept an error of 0.1 for secondary variables in the same field. Some fields I observe I can get photometry of 4 stars in the same field.

As for DSLR with my setup, a modified 500D with a 200mm f2.8 lens (about 70mm) I get good photometry to mag 8.3 with ISO400 and 20sec green channel only, unguided of an alt-az mount. I use a plan in APT that re-centres targets (with Goto++) about every 1 to 1.5 hours, as it does drift off centre without guiding on a low quality mount. 

Extrapolating isn’t easy as the magnitude will be a function of the camera pixel size, lens/telescope focal length and aperture.