At 4200, count on about 6 hours of paddling- 2 hours in to the Marble Canyon, an hour through the meat, and about 2.5 to three to get out (3+ if you have a spud boat). Add any play time, and then subtract your total from 4:30pm if you really don't want to paddle out in the dark. The paddle out isn't really that bad because the river gets a little channelized at low water. I think the lowest I've done was a 2200 cfs day trip in February one year, and we were done before dark. But, we didn't play much because of the cold.
Start early, but you could start as late as 10am.