Ultimately, "if you can outwork the rest of the population, you're going to get lucky," Cardone tells CNBC . And once you do start seeing success , don't change your mentality: "If you gave me $5 billion, I'd still be grinding tomorrow."

What sets the self-made millionaire apart from the rest of the population, he says, is how much he works : "Most people work 9-to-5. I work 95 hours [per week]. If you ever want to be a millionaire, you need to stop doing the 9-to-5 and start doing 95." That comes out to about 14 hours of work a day.

Entrepreneur and New York Times-bestselling author Grant Cardone was deep in debt before building his building his multimillion-dollar fortune.

He's not the only millionaire who says you should be spending more than 12 hours a day, seven days a week, pursuing your goals.

Gary Vaynerchuk, founder and CEO of digital marketing company VaynerMedia, says that if start-up founders want to make it, they should put in at least 18 hours a day for the first year.

In launching a business, "you have made a decision that does not allow you, in Year One, any time to do anything but build your business," the self-made entrepreneur says. "Every minute — call it 18 hours a day out of 24 — if you want this to be successful, needs to be allocated for your business."

"I think one of the biggest reasons so many people go out of business in the first year, first two years … is they don't realize how hard it is and how all-in you have to be," Vaynerchuk says.

Self-made millionaire and "Shark Tank" star Daymond John has a similar perspective. Ultimately, the secret to success boils down to one thing, says John: "Work. Bust your butt. Get up before everybody, go to sleep after everybody, and bust your butt. That's it."

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