Am I the only one who believes that it is bad for me to spend more than 20 hours -- and perhaps as little as 15 hours -- a week in the state?
Just because an activity feels really good does not mean it is good for you.
I guess I should explain a little how I came to believe that that too much intense absorption is bad for me. Let me start by quoting a passage (which I agree with) from another comment here:
>in order to achieve flow these three criteria must be met: 1. The goal must be clear 2. Feedback must be immediate 3. There must be a feeling that our capability is balanced to the challenge at hand.
The most important things for me to have made progress on in my life are things where it was not clear what my short-term goals should have been (although of course it was important for me to choose some short-term goals to work towards), where feedback was slow and rare and where I felt that the challenges I needed to surmount exceeded my capability. (I would guess that there are problems or concerns like in most people's lives.) In my past, I spent a great deal of time using the pleasures of intense absorption to distract myself from more important things.
Even after I realized that I was using programming and related activities like math and "programming-languages geekery" to distract myself from more important (but less enjoyable and messier) things, I did not know how to stop the distracting activities. In other words, I was addicted to the pleasures of intense absorption, and it took me many, many years to figure out how to extricate myself from that addiction.
If there are a lot of things in your life that are pleasurable -- hanging out with friends, sex or romance, physical exercise -- then maybe you do not have to worry about falling into the same trap as I did. I think that probably what really tripped me up when I was a teenager and a young man was that intense absorption was my only reliable or regular source of significant amounts of pleasure. Well, to be more accurate, satisfying my curiosity, a.k.a., learning, and intense focus on programming and related things were my only reliable sources of pleasure. (I have read that this pattern of only being able to take pleasure in one or 2 things and the consequent problem of addiction to those things is often a trap fallen into by people who were abused as children. I was abused as a child. Perhaps people who had happy childhoods are immune to the negative effects of spending one's days intensely focused.)
I still program for a few hours every week: I write Emacs Lisp code whose only user is me, which I do not get paid to write, and I am convinced it is not harmful to me. It makes me happy when I notice that one of my problems can be solved with code because coding is an efficient way to solve a problem. But if I ever take a job that involves programming full-time, I'll keep a sharp eye out for signs of a recurrence of my previous destructive / addictive relationship with the pleasure that comes from intensely focusing or concentrating. A big warning sign would be if I were to start again to neglect things that clearly should not be neglected like appointments at the dentist and such. Another (more ambiguous) warning sign would be a cessation of the process which has been going on for over 10 years now of my slowing increasing the range or variety of things I am able to enjoy or take pleasure in.
I decided to write this because in online conversations among programmers, we almost never hear (or read) about any negative effects of the "flow state". For me, there were significant negative effects: namely, I would have learned to deal with messy, low-feedback difficult situations at a younger age if I had not spent as much time in the "flow state". In other words, I got into the bad habit of using the flow state to avoid what I really needed to learn: how to make progress in messy "non-flow" situations.