For a lot of jobs, I would like to get away from the idea of fixed hours of work anyway. Put in a base line, say 1 - 4 Monday thru Thursday, so that you know when you can schedule meetings and see other people, and then let people put in the rest of the hours whenever they want.
For jobs that do require a fixed schedule, 6 hour shifts for 5 days a week would be the way to go for most. But I would leave it open for companies that wanted to do other organizations, up to 10 hours a day. Anything over 10 hours a day should require over time pay.
I would probably ban hospitals from running their stupid long shifts. Tired doctors and nurses are one of the more common sources of errors.
Jay