When younger, I certainly used to be able to manage substantially better than 5 hours, although "5 hours per stint, twice per day" might be accurate.
A great deal depends on whether one has been convinced that what one's doing is /worthwhile/, and (noting Bart's comment) I mean that in a fairly broad sense. There's also aspects related to different types of work, e.g. having to meet a 4-hour response time for a customer 150 miles away, working there 6 hours, and driving back (the American company I worked for did not look kindly on hotel stays since they didn't come out of a budget which could be directly recouped from customers).
These days, unfortunately, I'm not up to that and I also feel that I am not as good at giving setbacks the constructive attention they deserve. I'm reminded of the anecdote about Tom West in "Soul of a New Machine", where during a sustained work spell he found himself unable (by temperament as much as anything else) to do a relatively simple domestic repair: "threw the pump across the room and told his wife to call the plumber" were I think the words used.
Focusing on software development, I also find myself wondering whether the vast increase in the size of support libraries etc., and the consistently-poor standard of documentation ("why invest in writing manuals when the user can Google for peer support?") is something that affects all of us.
MarkMLl