There are many types of endeavors where I could list examples where it is time to call it to quits. But, specifically, here I am talking about: when is it enough stress?
I think it is when it interferes with your sleep. For example, waking up in the middle of the night without specific reasons or waking up exhausted regardless of the number of sleep hours.
Not everyone has the privilige to change something to improve their sleep, but if in your case you are able to, cut aggresively on the number of work related tasks.