That's a very subjective statement. I don't get even remotely burned out by 40 hours of work per week. I work every day of the week. I work when I want to. I don't have a family. I'm 32 years old. The organization of my life is by strict choice, and I love what I do, so I prefer to work longer hours. I'm more productive when I work 40+ hours.
I spend time learning, doing R&D / experimenting, doing product work, etc. The most fun I have is on the R&D. I've been doing it for 17 years straight and there's nothing else I'd rather be doing, I rarely suffer from exhaustion / burnout (and that's always when somethings fails, rather than feeling tired by the actual work).
This discussion is also going to vary dramatically based on the structure of your current life, including family / job / mood / etc. Adding further to the subjectivity of it.
I spend time learning, doing R&D / experimenting, doing product work, etc. The most fun I have is on the R&D. I've been doing it for 17 years straight and there's nothing else I'd rather be doing, I rarely suffer from exhaustion / burnout (and that's always when somethings fails, rather than feeling tired by the actual work).
This discussion is also going to vary dramatically based on the structure of your current life, including family / job / mood / etc. Adding further to the subjectivity of it.