The first one, which is the largest drawing I made, took about two months. Not necessarily always 8 hour days, but I did not count hours. I just knew at the time I had to get it finished on time, since it was sort of a comission.
Other than that, I really don't count hours. One time many years ago, my partner timed me on a easel size painting, and it surprised me that it wound up being 100 hours.