Although I use darktable since a few years now, I find the video lessons from @RileyB really interesting. I guess they will save beginners a lot of time.
5 s to never really finished. Sometimes it’s enough to click a few presets or even copy over the development from a previous image (Ctrl+c & Ctrl+v) and finetune it. Sometimes I come back to an image after months, because I discovered someting new or I try new ideas.
A set of presets for modules like highpass, local contrast, profiled denoise, sharpen and some styles especially for denoising saves me a lot of time. So on everage maybe 2-3 minutes per final image including sorting, tagging, which is quite timeconsuming.
darktable has a shallow learning curve, because it has so much to discover.
I don’t have one, if you ask for specific darktable moduls.
As hobbyist I only shoot ~7-9000 RAWs per year and I try to keep the number of shots as low as possible. People around me take many shots of the same scene. The advantage of RAW for me really is that I can concentrate on framing, focus and exposure. So I don’t need 20 shots of the same scene, which saves time and much more 
.