My limit is pretty high, depending on the circumstances.
Is my motivation high (love my job, big goal I am shooting for, feeling ambitious)? Is the pay good ("good" is relative) and is the need for money dire? Age and health play a role, as well as the rest of my life (do you have a partner, dependants, friends and family, hobbies that need your time?)
I find burnout is much more mental than physical. I can do something that's rewarding or chases an important goal for a long time and still have mental energy for the rest of my life. Chores can be outsourced if you earn enough.
I'm currently at a place where the personal value of my time isn't worth overtime (the mental energy it consumes and hours it takes from my family vs. the pay.) So we opt for me to work as close to zero OT as possible. OT pay would just shorten our time to FIRE, so we opt to take longer but have more family time now.