I've used Wakatime now for a few years (I highly recommend it), and have been going through one of the toughest projects of my life in the last 8 months. At one point I reached ~16 hours per day (for only a couple of days). This kind of output is not sustainable for more than a few days without incurring serious problems from it. I have now gone back down to a more normal 6-9 hours per day.