A lot of the answers are subective, that's b/c, as individuals, our tolerances vary so much. So regardless of what we say, you're still likely to experience burn out (if you haven't already). I know when I'm burned out when I have no motivation to sit in front of my computer. Unfortunately, the last time I felt that way I quit my high paying job and spent the next several months in a horrible place - mentally, physically, and emotionally. So at some point you'll realize your personal limits. With that said, my reason for posting isn't in regard to the before part, but rather the after - I've found that once you experience burn out, you're more susceptible to it in the future.

That's usually a bad thing; obviously you won't want to experience burn out again. However, sometimes it can have a positive yield in the long run. I've found that after my burn out experience, I've raised the bar in terms of the type of work I'm willing to do and how much crap I put up with. For example, I've always been passionate about home-run level customer service. As a consultant, that belief usually meant that I bent backwards anytime a client asked me to. I still have that belief today, but I tend to push back if they ask me to do things I won't be happy maintaining for the life of their product, or if it'll involve trying to hit an unrealistic time line, etc. So post burn out, I'm definitely more protective of my limits and happiness, I just wish I would've been this way sooner.