That's totally dependent on the artist and what medium they work in.
I can tell you that for me, a detailed pencil sketch can take 20-30 minutes in about a size 5x7", and won't include everything I wanted. This is after MANY many ball point ink practice sketches positioning the elements in different ways and point of view. I work off the best 'rough' to do the final design. Some of them are pretty rough too.
The inking of the sketch will probably take 40 minutes with three different width pens -- .005, .05 and 2. That's if I have no 'help' from the cats jumping up and shaking the table I'm working on, and my hand cooperates (I type all day) and my eyesight isn't getting fuzzy (I type all day off size 5 font pages). Wait 24 hours, erase the pencil guidelines, add more pencil work if you want to add something else.... Put it on the computer & 'clean it up' which takes 30 minutes usually because you're doing pixel work at high resolution. That's just the black and white side of things.
Color now.....oils? Acrylic? Watercolor? Multi-media? You'll have layers, drying time, adjustments...then when you put that back into the computer there's the fixing of the layers, adjusting color, etc. Unless you went directly to color addition by machine only, which is a whole nother story.