It's definitely getting tougher as I age.
Up until recently I did this flat-out for decades, six day weeks. Then five. Then I went to four. Now just three consecutive days of work makes me feel like I just can't barely even force myself out of the house each morning. It's grinding me down and my executive functioning in simply getting ready in the morning is becoming extremely poor -- as are my finances.
The bad news is, my government has raised the retirement age on me, so where once upon a time I'd be jacking in all this BS in six years from now, I'm now looking at not being able to retire until 68 or beyond. By which time, for all I know, I may only have a couple of years of freedom before I drop dead. It's not a great prospect.
On the other hand, I think if I could work from home at something I actually love and enjoy, I think I could work endlessly. I can spend hours and hours every day at my computer creating stuff and working on things, and I have to force myself to stop.
For me it's the going OUT somewhere to have to deal with a work situation that I hate more than the actual work itself. If I could work while staying in my own personal chosen environment you'd probably get seven days a week out of me.