I'm not really good for more than 6 hours of any sort of challenging mental work in a day, anyway. I also don't think that "hours" are a particularly good thing to track for software engineers -- we have ticket tracking and project management software that track effort already.
The bit about being rewarded with a Friday off reminded me of how I'd try to get my homework done during study hall in school, so I wouldn't have to do any at home. In the real world, the reward for getting your work done early tends to be more work. Maybe that shouldn't always be the case.