Brooks' was at IBM, they adhere to a 37.5 hours per week work schedule. Programmers who can produce 3-4 hours of real work per day are considered productive. Any more and you will start to burn them out.
I mean officially I was there 37, it was a good week if I was doing real, actual work for 20. Yes, I made up for it by working my tail off when it was needed.
We coders can be absurdly productive for short bursts, or be slow and steady. You can't have massively productive and steady for very long. In your early 20s you have a few years of this, but the more you push it then the more jaded you'll be later.