[struppi]

Just my opinion, but you should basically do what you like. I like developing software, so working for ~40 hours a week is OK for me. But I also try not to spend too many hours (per year) working for clients so I have time for my own stuff and also for hobbies (cooking, photography, sports, ...).

I also know that I am less productive when I am tired or overworked, so this is another reasons why I try to limit my time in front of the computer. For example, when it's possible, I simply take breaks when I am tired (http://davidtanzer.net/node/114 - though this is often not possible when working for a client on-site).

I could make more money by doing the same work I do now for more hours - but that would just be a brute force solution. What I currently try is doing work that has better hourly rates. I'll also try to build some passive income - that's what you mentioned.