If it's a hobby project I'm actually interested in, I can work all day every day. But if it's something I'm forced to do at work, 3-4 hours is the most I can do in a single stretch. If I can take a ~1 hour break after that and go to the gym or something, I can do another 3-4 hours. That's why WFH has been great for me; I can take a break as needed, and don't get bored to tears after a few hours in a cubicle farm.