This is so important to me. Working from home sounds luxurious, but when I tried it it felt more like living at work. You need to be able to draw a hard line between the times and spaces where you work, and the ones where your time is your own.
Tell your distant great grandfather, you know the small farmer in rural europe in the 1700s, that the cows just escaped the pen and see if he says "But its saturday evening, I'm off the clock".
As for the farm worker that is hired, you can be sure they don't bring the cows home with them.
He probably would if they were someone else's cows.
I think of it that way because work and life aren't two separate things—work exists within life—and neither do they balance equally. Work always demands a greater chunk of your time, whether it warrants it or not. If such a boundary is crossed and it is one that you find important, then you might recognise that much sooner than if you're thinking about balance, because you can start getting into specifics and where exactly you do draw the line instead of attempting to rationalise something you might not like.
At that point, it's not just work asking for longer hours, or flexibility on weekends; it's also stuff like your colleagues sending you messages and emails at 11pm or 2am, eating lunch at your desk, not taking holidays, hackathons (if those even happen any more!), being expected to be on call, leaving the office at 7pm instead of 5.30pm as the contract states, etc.
And after that, if you find work to becoming ever more intrusive, then the boundary system gives you enough info to recognise that this isn't really a 'work' problem, it's the same kind of shit you get in any unhealthy relationship and your working relationships are no exception.
However, others (and I put myself in this category) are pretty good at setting informal limits while being flexible about it. I tend to work 9-5ish at home but I'll run a daytime errand if I need to and spend some time in the evening if it makes sense.