When to
It is a tough decision. I got extremely sick when I was 21, almost 22. My PFTs were down to about 25%, sats in the mid to upper 80's to very low 90's. I was on oxygen for quite some time, but after a year and a half, I couldn't stand not working. My pfts improved some, to the low 30% area. I pushed real hard to go back to work because I felt like I was going insane just wasting my time away. Fast forward 9 years later, I switched jobs to someplace much less stressful (even though the same line of work), have stayed relatively active (but no actual excercise regimine), and I am finally making a good living again, which is very gratifying. My lung health has improved drastically (now upper 40's to very low 50's (53 is a best now)but now other issues are coming front and center. I work full time (over 50 hours a week after travel time), go to school full time, and still try to weigh in time for my treatments. I just found out I am cfrd though, and now i am starting to re-contemplate what I should be doing. I am in pain every day from severe GI issues and cfra, and my vit levels are dangerously low. Part of me wants to give up working so hard, the other part finds sanctitude in working and keeping my mind off my ailments.
I guess my point is this, if you really feel that retiring early is the way to go, you can definitely try it. It is probably better to try when you can do it on your terms than if you get extremely sick and are forced into it. If you are successful at becoming qualified, you can still twist some arms and re-enter the work force. Keep in mind though, the decision you make goes well beyond just reducing your work week, so there is definitely much to think about. The advice we all offer here is based on our own personal feelings, physical health, and mental ability to cope with the different aspects of our decisions. Only you can truly make this decision, however, so take your time and think it out clearly.
I wish you luck with whatever choice you make.