Here's how to know if you should turn your side hustle into a full-time business, according to a bestselling author and entrepreneur

As a serial side hustler turned New York Times bestselling author, Chris Guillebeau knows how to make a passion project lucrative.

"My whole life has been kind of a series of side hustles," Guillebeau told Business Insider.

The 39-year-old worked as a freelancer, overseas aid worker, and founder of several small business - often all at the same time - before becoming an author around 2010.

In early January, the Portland-based author and blogger launched a podcast and traveling workshop called "Side Hustle School" to help others create and grow their side hustle.

While many people set out to start a side project in hopes of generating extra income, others see it as an opportunity to eventually ditch their day job. That's OK, says Guillebeau, but you should always start your side hustle, well, on the side.

"I really, really encourage people to start their project from where they are ... Then, if it reaches a certain point in revenue - maybe it doesn't equal your day job income but it equals enough for you to live on and get by, even if it's not ultimately where you want to be - then it's kind of like you have your baseline," he said. "The right time to leave [your day job] is when you have something that's really working - it's not early when you have great merit or great promise or potential to it."

While Guillebeau says "there's an urgency" to create an extra source of income in today's economy, it's up to you whether it makes sense to grow that project into something bigger.

"[Y]ou can make that decision of 'OK do I want to go full time with this or am I happy with it at its current level - what do I want to do with my life?'" he said. "The point at which you answer that is when you have something that is approaching that level of sustainability."