I'm fortunate enough to work at a job that I love. Sure, there are phases when it isn't so great, and phases when I would give up my first-born to continue working on my projects, but on balance it's solidly in the plus column. Of the many things I could do in life to earn my keep, what I do now is very, very close to the top of what I'd like to be doing. That I get frelling paid to do it is amazing. And thus, there isn't a real limit to the number of hours I'm happily spending on my work.
What I don't get is that I'm not special. I don't have particularly unusual skills. While I have put in an unusual amount of time and school to develop them, there are lots like me. Most of them have also put in similar amounts of time and school, and they also love their jobs. The common thread among them is that they have taken to heart that old adage about investing in yourself.
So, to all of the people who are responding here with various versions of, "screw the man, my off hours are mine," I ask: why haven't you found something that motivates you? Is there nothing productive that you enjoy doing so much that you would do it for free? Have you not put in the time to figure out how to develop the skills necessary for that position, and work toward it? No? Then the best time to start is right now.