When does perseverance become overwork?

I think the balance is much more an intrinsic question than extrinsic. As long as you feel motivated and as long as you feel that's what you want to do, you can spend as much or as less time as it's needed. I think people can do much more work in half an hour of full focus than they can do in a whole day of just having to do it. This is obviously true for developers who found themselves just stuck in a problem and then they sleep and they come up with a solution. So this to me accounts also for everything else. If I wake up in the morning and I feel like, "Hey, you know what, this is one of those days and I'm going to just get stuff done."

Then I know that it doesn't matter if I skip lunch or I work until late because I don't feel it. But if I wake up and I say, "You know what, it's not the day. I don't try to overdo it." Because if I try to overdo it'll sling across the other day and it will drive me through and there's no point. That's when it starts to get scary because you do things because you have to instead of because you want to.