Actually, I wonder at which point you guys notice that your productivity drops. Very often my productivity drops noticeably after 7 hours, already, but it can come back in the evening.
Also, it often drops after lunch, but this also depends on when we have lunch. It's significantly worse if we have lunch around 12:00 than around 13:30-14:00. Our research group loves to go at 11:30 (!!!) which destroys the whole day for me, so a few of us have just stopped eating together with them.
I telecommute as well and basically have the first six hours of everyday by myself, so I feel very productive during that time. After lunch it drops some, but luckily that is also when meetings are so I feel it doesn't matter as much. If I still have time left and don't feel very productive (which happens maybe one day a week, sometimes more though) I usually have some work that doesn't require much mental effort (like administrative or light maintenance) and work on that. If I couldn't define my schedule like this it would probably be harder.
"Seventy-five percent of Dutch women now work part time, compared to 41 percent in other European Union countries and 23 percent in the United States"
"Twenty-three percent of Dutch men have reduced hours, compared to 10 percent across the European Union and in the United States; another nine percent work a full week in four days."
I've considered it, but I know my productivity sags at the 8 hour mark as it is, so I don't think it's a good fit for me or the company.