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.
Not sustainably... I've done that myself: it's hella fun but towards the end of the week the code stopped making sense and had to sleep for a whole day. WakaTime is very accurate so my daily average is only 4 hours of actual typing per day:
