Yikes, this is a loaded question.
If I worked a normal shift, with two days off per week (yeah, I wish) and I could actually sleep when it's dark, I would probably sleep 7 or 8 hours.
As it is, my schedule is insane, I work from 6-18 hours a day, I have no set day off, and it's never more than 1 per week, and I work nights.
I sleep from 4-12 hours per day.