@Frost-Drive You've gotten some great advice here. Before I read the other comments, I too was thinking, "Well, it depends what stage you're in." The initial creativity that goes into concepting and thumb nailing a piece requires a lot of extra energy.
Also, I find that if I don't take breaks about every two hours, I literally start to lose sight of the big picture and I end up wasting a lot of time. But 90 minutes sounds about right.
And I have never been much for mindless activities. I spend my free time on friendships, listening to teachers/speakers while doing housework, and taking walks to empty my head and release the tension that comes from being slumped over the tablet for too long. Those things are a good counter to working alone all day inside my own head. Each person will find their own balance.
Then again, my goal isn't to be a human assembly line, because I don't think that's what makes a good artist. My goal is to make good art (and hopefully get paid for it eventually), and that does require getting more work done.
That said, I wasn't working fast enough, so last week I did exactly what you suggested about getting my life out of balance: I participated in Childhood Week, and I only had roughs going in, so I ended up working from early morning until about 9:30 every evening to finish each piece. Nothing else in my life got done except for the bare minimum required to put meals on the table (for my husband, because I would have just put tuna on a salad and called it good). At the end of the day, I collapsed, got up early the next morning, and started again. And every day from about 11:00-3:00, I had that kind of near panic that children feel when they start to run down a hill and get out of control and don't know if this sensation will end in exhilaration or a trip to the ER .
Although I didn't know how much work it was going to take to bring a piece from roughs to finished each day, I did know that I was setting a tough deadline for myself, and I did it precisely to force myself into production and greater efficiency. I work much too slowly and am too much of a perfectionist in ways that don't make for better finished pieces. Doing a drawing challenge forced me to finish work extremely quickly and post it whether I liked it or not. And in the end, I posted 6 out of 7 days (for one, I just couldn't get the rough to read well and will have to go back to it). Out of the 6, I really like 2, 2 others have potential but maybe need a little more work, and 2 I dislike but posted anyway because that was the challenge.
Could I go on like this? No, because my household is a disaster even after just one week and there are things I simply can't put off anymore. And my personal life needs some attention as well. Not to mention that I moved all my English students' lessons to early this week. But I highly recommend doing a challenge now and then! As a limited experience I think it was very valuable, because it "velocitized" me. Now I hope I can go back and finish a piece in a few days to a week instead of half a month. I streamlined my process. I looked at my pieces compared to those I most liked in the challenge and noted the differences. I made a list of aspects of illustration that I want to go back and study to improve my work. None of this would have been possible in the same way if I hadn't forced myself to finish work very quickly and get it out there.
In the end, I think the secret to drawing all day is motivation combined with a workable routine. Motivation may be love of the work, need to earn money, or setting yourself a challenge in which you are accountable to others. For the routine, I recommend thinking about what will bring you both enough hours, but also focus on what makes your work better. Only you will know the right combination for you, but listening to others' experience helps.