What a great question.....unfortunately, not a question with a quick, simple, or easy answer.
I no longer teach in the regular classroom (I do, however, co-facilitate ISW workshops), and so I can only think about the question from the viewpoint of my current work.
There is a balance point, and I have no idea what it is.
I think the balance point changes not only with each person but with the combination of players of a given project and also as a result of the project itself.
Passion and energy commitment may well connect to the number of real or perceived obstacles in the way, the reception of one's ideas and work by colleagues, the willingness of one's institution to follow a particular path.
Burnout (whether or not one is a teacher) remains a very real possibility.
But it can be hard to maintain the passion when you encounter groups or individuals who don't want to engage, learn, participate.
I don't teach full-time and I get opportunities to engage in project-based learning development. I find the variety helps me stay balanced and ensures that I go into teaching sessions with enthusiasm (most days that is ;-)
The thing that keeps me excited about working with practising teachers is the increase in energy and passion as they develop over years the habit of continually taking on measured challenges. Certainly, I have seen this orientation derail people and move them into the zone of major stress when they take themselves TOO seriously and expect perfection the first time through.
My experience in the past tells me our students APPRECIATE efforts to explore different forms of instruction, taking personal risks, because they recognize that at the heart of it, we know our curriculum, are not afraid of making some errors along the way, but are clearly committed to their learning. When the students see me getting formulaic, overly predictable, or worse, when it all starts to look the same to ME..I know that I need to launch into something more challenging - for the students and me.
I think burnout comes when we become too perfectionistic, when it all starts to look the same, when all risk is removed from our agenda, and we get too wrapped up in the tight fidelity and control of our curriculum areas, or just as badly, the purity of our pedagogy - rather than being responsive to what and who are students are... my thoughts..
That said, the first year prof often needs the basics - to build a safety zone initially around a few simple instructional basics. I always need to remember that the risk and responsibility in teaching a hundred undergrads is daunting enough!