I usually spend only 15 minutes - 45 minutes on a "sketch". This drawing took about 5 hours of sitting time. My realistic drawings take anywhere for 3-14 hours depending on the level of precision required.
I didn't know I could draw like this because I had been "sketching" my whole life until I encountered Tony Ryder's description of the difference between sketching and drawing. Then, I took Natasha Mokina's classical drawing class where we studied from casts and life. That classical training dramatically changed my view of drawing and my work noticeably improved under her mentoring.