Hello,
When SU takes longer than I expected,and when I did not save (especially for intersections), I open the autosaved version in a new window. I kept the autosave every 5 minutes in preferences, even if it's annoying while working on 100Mb+ files. This way I can work on my model while the first one is still working. (SU only uses one Core, so it shouldn't slow your PC even if it's working)
You'll then get an AutoSave_AutoSave_filename.skp and so on.
Just check the last autosave file's time or even open it in a new window like I do to check what you would lose in case you chose to force quit an hanging SU. Sometimes it's faster to lose 5 min work and find a workaround, SU beeing able to hang for hours !