[pdimitar]

Of course I am not talking about barely working at all or some miniscule amounts of hours, come on now.

It's the latter from your comment -- I disagree where the peak is for software developers. But I can't give you absolute numbers that I can claim are true for everyone. There's no such thing. I am a proponent of the per-individual approach.

That being said, if we try and devise a good median number -- I've read quite a few articles several years ago that claimed that 27-32 hours of work are not only the peak of the programmer's productivity but that's also the working schedule that makes people want to work more in their free time. So in terms of your "free 20-30%", for people like myself a 30-hour work week would likely result in free 50% once every several weeks.

Not feeling forced into unproductive schedules gives your brain and psyche a good leisure time during which you occasionally find yourself thinking about the work problems in a chill state -- and it's also often the case you reach for your laptop on the couch to try and solve them outside of the working hours.

Again, it's subjective. I knew several people who worked best when they were given tight schedules and impossible work hours. I have to somewhat gloatingly add that all of them ended up in hospitals and subsequently became very strong advocates of low-to-medium working hours. So IMO the "punch the sun!" people don't live a sustainable work life.