What are the limits on a teacher's time?

A teacher at "Education Nation" suggested that union rules regarding the teaching day limited her ability to meet the needs of her students and she just wanted to "do my job." She wondered why she wasn't allowed to bring students in on Saturday to do extra tutoring with those who needed it.

My simple question to her is: Where do you put limits on your time? Most teachers I know, myself included, put in time beyond the contract. Today I worked one hour forty-five minutes beyond my contractual obligation. On this past Sunday I spent three plus hours in my classroom grading papers and planning. I also scored papers at home on Saturday and Sunday. I brought home more papers to score tonight.

When I was a young teacher, I put in many more hours than I do now. Of course, I was living away from family and friends, and had no social life to speak of then. Now I'm older and have a wife and son. My energy level is not what it was. Tell me, what is a reasonable expectation of my time as a professional? Do I reach my limit when I drop dead?