This depends totally on your imaging scale (that would be measured in arc seconds per pixel). Image scale depends upon your focal length and the pixel size of your camera. The acceptability of an image also depends upon how closely you look at the results and the kind of reproduction scale you use to display your images.
This past week I was imaging with my 9.25" EdgeHD using Celestron's 0.7X reducer (effective focal length approximately 1640mm). However, I was using a camera with small, 2.4 micron pixels (the QHY5III-178C) so my image scale was just 0.3 arc seconds per pixel. My mount was a high-end Astro-Physics Mach1GTO that had been polar aligned with a PoleMaster (probably within a few arc minutes of the true, unrefracted, north celestial pole -- or about as good as you can do without doing a lengthy and careful drift polar alignment).
So, what was the length of the UNGUIDED exposures that I was attempting? Only FIVE SECONDS, and even that was difficult. Basically, when you are working at the kind of image scales provided by an SCT you NEED to do guiding (and best case with an Off Axis Guider, as a separate guide scope will probably only be good for relatively short exposures, maybe a few minutes). Of course, if you turn to something like a HyperStar (f/2) then your image scale will be reduced to a much more manageable level. Now add in a camera with larger pixels (e.g. ASI1600 with 3.8 micron pixels) and you could be imaging at something approaching 2 arc seconds per pixel (about six times as large as the example I gave with my EdgeHD).
Now, given that 6X relaxation in the image scale, you can probably do unguided exposures for a minute or two or even longer if you aren't particularly picky. By "picky" I mean you pixel peep and still want round and small stars.
What kind of difference does this make? After rejecting about one half of my subs for either star size or shape I was able to get an image that had a Full Width Half Maximum (FWHM, a measurement of the diameter of the star's image) of only 1.85 arc seconds in broadband color. Of course, not all of my problems were related to the unguided imaging, since the seeing conditions also matter (a lot). Having said that, if I had attempted exposures as long as two minutes I'm sure that I would not have gotten a single sub that would have met my criteria (round and small, and I imaged over three different nights and collected well over two thousand subs).
So, the image scale matters, the seeing conditions matter, and the eye of the behold matters. Frankly, for a CPC 9.25 I would not attempt unguided imaging unless you were using something like a HyperStar or had a camera with VERY large pixels (or both). However, if all you want is a run-of-the-mill image that will only be viewed at a very small size (casually) then practically anything is possible. And, of course, you can get lucky now and then so YMMV. In fact, that's one of the reasons why ultra short DSO exposures are often termed "lucky imaging" (partly due to the seeing conditions and partly due to the unguided tracking provided by the mount).
So, yes, IMO "an exposure of 120 seconds
an [is an] unrealistic expectation with my [your] setup."
By the way, if you want to look at an image of M51 that has a FWHM of under 2 arc seconds then here is something that I did guided on a Mach1GTO using a 660mm focal length refractor:
Edited by james7ca, 15 April 2019 - 10:25 AM.