Personally I am more of a sprinter, and currently my target is 4-5 hours of max intensity work in the morning, with free afternoons, 6 or 7 days a week. No distractions, no social media, no people around, avoiding browsing the internet for fun from waking up to 1pm.
Free afternoons feel like every day is a holiday that can be filled with whatever I fancy, though sometimes morning work motivates me so much I might do more work in the afternoon. I might need to pace myself, like that Hemingway quote in TFA.
It is incredible how productive one can be with just 20h of consistent work a week, with no bosses or bullshit meetings. I'm still working on the consistency part :)
On a motivated day I can work as much a full day of work (7h), most of the time I'm spending a couple of hours on it. Everyday (mostly). I find it easier to be consistent by having a day job that firces me to wake up in the morning than when I have a full empty schedule.