For me, I don't want a "fun" job. I pretty much have the best gig I can imagine right now, and I am well compensated for it (99% stress-free, but that 1% is where we really earn our money). I could see cutting down to half time if it were allowed, and I'd still get the benefits (health care at retirement age, pension, immediate withdrawal from my TSP, etc), but I can't see quitting something that pays this well, is this "easy", and gives me time to work on my own stuff (I spent a few hours hand quilting tonight while I had to sit in a certain location to push a few buttons, wait, push a few more buttons).
I've been working since fifth grade, and having a job is just not "fun" to me. Fishing is fun, quilting is fun, hiking is fun, going to the theater is fun, travel is fun, mindlessly doing nothing is fun. Work is not fun. I can have fun at work, and often do, but work is work. I'm one of those people who enjoys things most when I don't have to do them. Loved raking leaves as a kid, but the second I was told to do it, or how to do it, I hated it. I love making quilts, but if someone wanted to pay me to make one for them, I'd lose interest, especially if it wasn't my taste (it took me forever to make a simple quilt for donation that had pink as a main color. I hate pink and it ended up taking me 10 times as long to finish that quilt, even though it was about a tenth of the work when it came to cutting, sewing, etc. I just didn't enjoy it.)
So, at least right now, there is no magic number. I want to do what I do, as long as it stays tolerable (decent boss, and not exposed to too much of the political bs, no one stabbing me in the back to get a promotion, great schedule, etc). Then I want to retire and not have to try to earn money ever again.