So if your main job is coding, you can probably spend about 6 real work hours on that. This leaves you with another 3 hours to spend on stuff like email, or writing or whatever. Certain types of hobbies do count into this as well.
I'm not sure who did this research, but I can tell you from personal experience that this is simply not true. Maybe I'm a freak of nature but when I was first learning how to code I would hold one man hackathons in the summer from the moment I got up until deep into the night. Similarly, when I was younger and played a lot of RPGs, I would play quite literally all weekend, pausing only for sustenance and a little bit of sleep, and usually multitasking on the food part, far surpassing 6 hours of focus time.
Most folks cannot sustain focus for super long periods.
I suspect the "must work 24/7" thing is probably rooted in the fact that some folks are just like that and those folks are often crazily successful. But I think the inference that high number of hours worked is the thing to shoot for isn't really accurate. Some folks don't need a lot of sleep. Some folks can focus for long periods. Someone who has both traits may well be brighter than average.
Their phenomenal success isn't necessarily rooted in any one thing. Putting in the hours if you aren't that bright and actually need more sleep may be a recipe for disaster instead of a recipe for success. However, people like Bill Gates do tend to work crazy long hours. You can't ask to be born with his brains -- you have whatever brain you have -- but you can try to emulate his work habits and hope that's the most important piece. But I see no reason to believe it is. Perhaps long hours are a good idea. I just have yet to see good data which proves that. I have seen more data that for most people job performance improves if they cut back their hours and are working when fresh and focused.
Are they? What if you just don't notice the unsuccessful ones; they're at home spending ten hours a day drawing pictures of butterflies, or they're down at the gym obsessively lifting heavy things over and over again, or any one of a number of other obsessive activities that don't bring success.
Nowadays it's a bit different. My body just can't take the sitting in front of the computer for so long. Maybe it's a good thing though, but it really makes me want to think about how I spend my time on the computer and seek those creative hours when I am able to get into the zone and just code.
Usually I find myself doing from anywhere to 2 to 4 hours of concentrated creative work a day, and that's it.
As for me, I've noticed the amount of time I can focus on something go down a little over the years as well, though I still occasionally get inspired and hyper focus like I used to.
One of the defining characteristics of ADD/ADHD is ability to focus for extended periods. The main reasons ADD gets a bad rap is that people (often children) are either focusing on something that adults don't want them to focus on, or there is nothing interesting that holds their attention so they become easily distracted.
But I've really only gained 2-3 hours over one 7-8 session. 12 or 16 hours is completely unsustainable over more than 2-3 days.