To be fair, I worked nearly full-time for several years (37.5 hours per week, government job which also had a ton of paid time off) and then I've been back in school for the last couple years, which pays me a stipend. I also work part-time as a software developer. But, basically, I just don't have a ton of expenses (I don't own a car, I have lived in cheaper areas, I have no children). I will likely end up with a full-time job in the near-ish future. If I had it my way, I would always work part-time, but it is quite difficult to find good part-time work, and that is sad.
Full-time is generally 40 hours per week in the US, although there are always exceptions. The vast majority of "office" jobs, however, insist on 40 hours per week.
It helped that I could truthfully claim that I pursue a college degree on the side (distance learning, so it's possible to schedule work and study), but it went through with surprisingly little effort.
Meanwhile my coworkers are wondering that this is even possible (and sound mildly envious, in a good way).