Figure that you'll be out there 12-14 hours total, so if you leave really early you should still be there in daylight. If you are faster then there's no problem at all. If you can't finish in 14 hours you're pushing it with the daylight. 10 hours is a pretty good time, and that's the best I ever did. That included a 4.25 hour first 100, and that's really fast. Add in the breaks and I didn't slow down too much on the second hundred. My time included all breaks, so I started at 6am and finished at 4pm. I'm happy to finish a single century in that time now.