I think the upper limit, really is consecutive hours before you need a good rest to be able to start over. If you can do that over 4 days, you can do it over 5. A standard 7-8 hours a day allows both for breaks during the day, and good rest overnight so I don't believe that it is "ridiculous" to think that it is sustainable over 5 days. (Happy to be proved wrong).
As an anecdote or personal experience, I think we've all been in offices (all/most offices?) where people were not exactly flat out all day long...