That’s great! I love hearing that people are learning a lot from my course 
If I am just editing RAW photos, generally from 30 seconds to 5 minutes. Of course, that is just an average. Some images will take longer.
I find that since darktable has so many different modules to play with, it is easy to spend a long time editing. Following a process helps a lot. And it looks like you already have one.
I will echo what others have said. When it comes to quick editing, presets and batch editing are you friends.
Like @patdavid already mentioned, I also find that I have a set of images with the same lighting conditions. I figure out how to edited one them (maybe try it a few different ways), then copy the edits to the other photos. Then I move on to the next set of images and repeat.