Let's start by doing the math on how much 'free' time a person has.
(168) A week is 7 days, 24 hours a day, so 168 hours a week.
(-56) Each day we have to sleep approximately 8 hours (give or take, I'm going to assume an average of 8). That's 56 hours a week.
(-21) We also need to eat approximately, if not more than, 3 meals a day. There may be some food prep, whether at the time of eating or before hand, so each meal is an hour.
(-7) We'll also need to get ready every day, such as shower, shave, brush teeth, get dressed, etc. We'll be generous and say that takes an hour a day.
(84) We're down to 84 hours per week of awake and active time.
(24) Now if we do that on a daily basis let's take a breakdown of just one day.
(-3) Eat three meals.
Neither of these take into account transit time to get to and from work, which, more often than not, isn't productive time to either personal or work.
Humans needs time to rest, play, entertain, or socialize. They can't just work non-stop. The key is to find a good balance. I believe half of a persons daily or weekly time should be sufficient, so we're looking at about 42 hours a week or a maximum of 6 hours per day. Given that we typically only work 5 days a week that puts us at 30 hours a week. Any one of those numbers would suffice for finding a good balance. Anything more, though, and we're trading personal time for work time, something most people can't keep up forever.
Lastly, a number of studies have shown that working beyond a certain point and people get drastically less productive, so those extra hours quickly turn to mush.