Is there any limit to the number of hours that can be expected from a salaried worker?

I mean, can a salaried worker be expected to work 24/7 and if they are unable or incapable, can they be fired? That s obviously not my case, but I question what is reasonably expected of me.

I am "contracted" to be onsite from 8:05-4:45, with a 30 minute lunch (so 8 hours and 10 minutes a day). But there are obviously expectations beyond that 8 hour day. I figure that, to meet all of the expectations, I would need to work 72 hours per week (a full 30 hours above my "contract" hours).