4 hours, in my experience, is the absolute maximum that a motivated person can work on a heavy task in a day. I can't cite any study for this, but it's a number that I've seen used by really solid teams, internally. Billable hours, of course, exceed 4 hours-- but that's because billable hours includes "boilerplate" work.

4 hours is A LOT. You can get a lot of stuff done in 4 hours. 4 hours x 5 days a week can be sustained long term as well provided that the remainder butt-in-seat work isn't too draining/demoralizing.