Well over 40 million people in the U.S. have a side hustle, in part because you can set up your own small business in less than two weeks

Deciding when to quit your full-time job and turn your side hustle into your day job is hard. I should know: That's what I did when I launched LogoMix. (More on that in a bit.)

If you're unsure about whether to take the full-time entrepreneurial plunge, ask yourself these questions:

1. Is my full-time job still challenging and exciting?

The grass always looks greener on the other side of the professional fence. And it's always tempting to be your own boss instead of working for someone else.

But at the same time, you carry the skills you gain from a challenging full-time job with you forever. If you're still learning, growing and excited by what you do, think carefully about whether staying for six months or a year might pay off over the long run in terms of skill, experience and connections.

In my case, I enjoyed what I did in my full-time job, but I saw few things over the next year that would excite or challenge me.

What I knew would be challenging was turning my side gig into a larger business. I knew, unlike my full-time job, it would take everything I had -- a prospect that definitely sounded exciting.

2. Is my side gig making money?

Generating revenue is one thing. Generating profits is another. If you spend more money running your side gig than you earn, you haven't figured out a path to profitability. That's especially important if you plan to bootstrap your startup by using revenue to fuel operations. If you're not making a profit, you'll soon run out of money and be out of business.

When my side gig that eventually became LogoMix started making money, it wasn't enough to support me. But the results proved the potential -- that building out operations would generate greater revenue and the business model could reliably turn a portion of those revenues into profits.

Determine if your side gig is truly making money. Then, forecast the amount of money you can generate if you turn your attention to your business on a full-time basis. Is it sufficient to tide you over until your business thrives?

If it's not, stick with your full-time job until you know you're ready.

3. Have I found a sufficiently broad market?

The idea of an enabling customer is enticing: That one big customer that will generate enough revenue to truly launch a business. 

Unfortunately, startups find it incredibly difficult to actually land an enabling customer: Deep-pocketed clients naturally tend to choose established companies, not side hustlers. (Plus, if one customer accounts for 90 percent of your revenue and that customer leaves, then what?)

What you need is a broad base of potential customers -- a base of people with the same needs, pain points and  problems that need to be solved.

Find those customers and you can build out your operations. Refine your service model. Adapt your products and services to the market's needs.

If your side hustle is a niche business, make sure you can expand beyond that niche to build a larger and more robust business.

4. Can I see a clear path to grow my side hustle into something big?

Not every business is easily scalable. (For example, if you directly provide a service, your earning potential is capped by your capacity.)

On the flip side, I tested the market and found small business owners needed an easy, affordable way to build their brands. I spent a lot of time automating certain functions to make the business easily scalable.

Not long before that I had sold my previous company to HubSpot. One day I told Brian Halligan, the HubSpot CEO, that I saw a path to a few million dollars in revenue with my side gig. He looked at me and said, "Then get out of here and do it!"

Within a year, we had hit $3 million in revenue and were still growing fast.

Make sure you see a path to the growth and profitability you seek. If you don't, keep grinding away. In time, it will appear -- and if it doesn't, you'll be glad you kept your full-time job until the moment you did see a clear path to building the business you want to build.