Hi Lilbo,
This week, after today, I'll have done about 15 hours. I'm part-time and teaching for the best part of three days a week, even so, I'd love to fit more in, but I fnd I have a productivity cut off point in each day, and need to work in two hour bursts, fitting in two, maybe three bursts in a day. I can work more solidly if I'm just reading, but I'm writing at the moment. During the holidays I work more, for example, this summer I wrote a a paper, and worked, what felt like, 24/7 on that for about three weeks, but then I took two weeks completely off, which I needed badly by then (the only time off I've had since last Christmas). The rest of the summer I worked about 35 hours per week, I wish I could concentrate for longer than that! However, when I do work, I am totally switched on to what I'm doing, I tend to get lost in it.
Oh, the hours I've given you apply to researching and writing, not to library visits and emails etc.