Andrew:
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.