Let's put it that way....
One of my mates, who is a genius[*] (and a nice guy), once told his boss during a similar discussion:
"Look, I can be a genius not more than eight hours a day."
I think that for me the limit is about six hours, but I start at a way lower level of intell-uh-djence. So working only part time (young kids & stuff, and stupid "back to the office at least 50%, no matter if the colleagues you are working with are in the same city, or even state") actually is a pretty good deal for my company. I'm pretty sure I achieve pretty much the same as when I was working full hours. Except when reading ElReg[+]. Ok, back to work, slouch.
[*] not the fruity tat merchant type, but a guy who actually has both a high IQ[#] and can actually apply his genius-ness (I'm not a genius, so I'm allowed to make up words) to real world problems - yeah, he is a genius (or at least quite a bite more intelligent than I am).
[#] he kept quite stumm about it, but we found out, his wife (who is a physics professor) mocked him about it in front of his mates (many do have a PhD in a real science, so not humanities or arts, so I guess it is pretty ok to mock him about it in that group - it is not bragging, that's what I mean)
[+] I am keeping abreast of recent internet technology, as the PFY once put it... and if reading ElReg is not part of your job you got the wrong job.