Mark Zuckerberg and his wife, Priscilla Chan, have welcomed their second child, a daughter named August. So, the CEO and founder of Facebook is taking two months of paternity leave. (It's staggered, so that he'll be out for a month, back for a month, then basically take the month of December).

If Zuckerberg is willing to step aside for even a short while, many of the other modern giants of entrepreneurship have a different attitude when it comes to work-life balance. His COO at Facebook, Sheryl Sandberg, famously opined that the concept doesn't even exist--and she's not alone.

"People who talk about work-life balance, I think that's a mistake," says Steve Blank, the serial Silicon Valley entrepreneur and author who "decided to go home" and raise his kids after his last company went public. "I think if you're going to invest in a family, it ought to not be balanced. [Your investment] should be in the family."

Here's how some other entrepreneurial icons opined on the subject--or else displayed their ideas through their actions:

Elon Musk

Musk isn't the wealthiest entrepreneur (yet), but he's one of the most interesting of our time, between Tesla, Space X, his "hobby" the Boring Company--and don't forget his role at PayPal.

He's been married and divorced three times, including twice to his second wife, English actress Talulah Riley. He and his first wife, Justine Wilson, lost their first child as an infant, but they have five sons--twins and triplets.

Writer Eric Mack says he was struck by a comment Musk made at an event in 2013, when he insisted he manages work-life balance, but also says he manages to keep working while he's with them: "Because they don't need constant interaction, except when we're talking directly," Musk said.

Interesting. Couple it with this description of what it takes to become a billionaire, written on Quora by Musk's first wife, Justine. She never mentions her ex-husband by name--but still:

"You're determined. So what? You haven't been racing naked through shark-infested waters yet. Will you be just as determined when you wash up on some deserted island, disoriented and bloody and ragged and beaten and staring into the horizon with no sign of rescue?"

Steve Jobs

As far as I can find, Jobs never talked specifically about work-life balance; if he did, he didn't quite become part of the canon, so to speak. But the late Apple co-founder, whose vision resulted in the development of the device you're probably reading this on, had quite a few other pithy comments.

Among the most-cited: "Your time is limited, so don't waste it living someone else's life."

It's a nice quote, often interpreted as being about not following someone else's plan for you. But maybe we need to look at it in the context of both Jobs's professional success and his personal failures. By all accounts, he was terrible at work-life balance, to the point that he denied paternity of his daughter, even as he named his computer after her.

So if you think about the quote in that light, it could also be seen as about not putting others' needs ahead of your own. Isn't that the definition of selfishness?

Richard Branson

Of course, I don't actually know Richard Branson, so I can't say for sure how well he pulls this off--but he at least makes an effort to promote the importance of work-life balance.

The 67-year-old serial serial serial entrepreneur has been married almost 30 years, and has been with his wife, Joan Templeman, for much longer. They lost a baby daughter who was four days old in 1979, and have a son and a daughter.

"I wouldn't be where I am today if it wasn't for my family," he wrote last year in an article entitled, aptly enough, The Importance of Work-Life Balance. "When I was young and just starting out in business, my parents encouraged me and supported my entrepreneurial spirit. I've tried to be similarly supportive of my children, and I make it a priority to spend time with them and my wife, Joan, every single day."

Sara Blakely, founder of Spanx

"I just layered the full-time job of being a mom on top of another full-time job at Spanx and then wondered why I was so exhausted. I started to notice changes in my health and couldn't think as clearly. I feel like this happens to a lot of women. We just assume we will figure it out. There is no manual, and no way to truly prepare. I am still a work in progress."

"I believe that being successful means having a balance of success stories across the many areas of your life. You can't truly be considered successful in your business life if your home life is in shambles."

Gary W. Keller, founder of Keller Williams Realty International

"Work is a rubber ball. If you drop it, it will bounce back. The other four balls -- family, health, friends, integrity -- are made of glass. If you drop one of these, it will be irrevocably scuffed, nicked, perhaps even shattered."

Jack Welch

"There's no such thing as work-life balance. There are work-life choices, and you make them, and they have consequences."

Ev Williams

"Failure of your company is not failure in life. Failure in your relationships is."