How many hours will he need to allow to paint an 800 square foot area?

It will depend on what you are painting. 800 square feet of floor space with a roller would take about an hour to an hour and a half, though you can go faster if you really push it.

If you have to prep, mask off and cover floors to do ceiling or walls, as well as cut in with a brush, closer to an three hours. Each door or window adds about 15 minutes to the paint time. Each room involved will take another half hour or so. And you'll need another half hour for each paint color.

I've put up 18 gallons of paint in three days, which is about 7,200 square feet. That was drywall primer and two coats of paint. Three different colors plus the primer and ceiling white. And there was absolutely no masking to be done, no furniture, baseboards or floors to worry about. Just the cutting in of corners.