Success can hit you like a ton of bricks.

For these entrepreneurs, it was partly a realization based on circumstances and partly a moment in time when they shifted their thought process.

For anyone pursuing success, you can learn from these examples as a guide to know when you have reached a success milestone.

1. When a company survives beyond your involvement

"I've run five start-ups and would describe two of them as successful (my first and likely Magnetic) and three of them as 'meh'.  I'm not sure I've ever felt the inevitability of success until the end. Even with Magnetic, a company that's grown from $1.5 to $100+ million in revenue in 5 years, there's no predicting the future. We've all seen companies grow only to implode. I strongly suspect that my unease and lack of desire to declare victory helps us become more successful. And so the only time I've been prepared to declare a business successful is when it's been sold, the investors have made a return they are happy with, and I can stand back and see that what we built is going to survive beyond my involvement." - James Green, CEO of Magnetic

2. When you learn how to live

"Actually, I didn't for a very long time. I was a very insecure child who never received any encouragement from my father, an immigrant from Hungary newly arrived just as the Great Depression was starting. My father had no success or confidence himself, and gave me none. Even when I graduated from Harvard Business School at the very top of my class, I still had no confidence in myself. But like the man who said he became an overnight success after working his butt off for 40 years, I finally knew I was a success in leading the investment bank I owned when my closest colleague told me about a meeting he had just had with Ace Greenberg, the brilliant chairman of Bear Stearns. When Ace learned that my firm, with only 25 people, had earned in excess of 50 million dollars that past year, he responded, 'Wow! That's great! It's way more than my major firm earned.' It was then that I finally knew, or thought I knew, I was a success. However, while I believed I was a 'success' because I was making such a great 'living' (financially), the most important aspect of life which has made me a real 'success' is that I have learned how to 'live'. I have a warm, wonderful loving family that I cherish and the opportunity to have given many millions of dollars to health research, education and other institutions that have helped improve the lives of so many others of my fellow human beings. That's indeed my most treasured 'success'!" - J. Morton Davis, Chairman of the Board, DH Blair Investment Banking 

3. When you pursue a personal passion

"My personal eureka moment came when I stopped worrying about success altogether and pursued my own passion. My vision is for a world in which technology eliminates heart attacks, strokes, heart failures. That this resonates so strongly with the many people whose lives Qardio touches makes my work all the more rewarding." - Rosario Iannella, the Chief Technology Officer at Qardio

4. When uninvited job applicants just show up at your company

"I think the key to success is doing things differently--something we really strived to do in starting Maxus Australia. After about three years and countless hours doing what we thought was 'different', things finally started to click when uninvited applicants began turning up, looking for jobs we didn't have. We asked them why they'd come and they would say things like, 'this place just feels cool, can you call me if something does come up--anything?' We also started getting asked to pitch for projects that weren't really even the domain of an agency like ours--making TV ads, building creative tech solutions, etc. People started coming to us for our perspective - to tap into the way we think, invent and solve problems, but also trusting us with the delivery of these new ideas. It was scary, liberating and insanely satisfying.?" - David Gaines, Chief Planning Officer at  Maxus Americas

5. When you find real joy and gratitude

"I knew it when I realized that success was really about how much time you spend living in a state of joy and gratitude. It's more about your emotional state than accruing money. And the things that bring me the most joy and gratitude are spending quality time with my life partner and two boys, inspiring my employees to live their potential, and developing products that truly delight and inspire people. Those things define success for me, so when I realized all of them were within my own power -- that's when I knew success was inevitable." - Colin Guinn, Chief Revenue Officer at 3DR

6. Your first day in a new office

"I was born and bred in the UK and built a successful communications recruitment business from the ground up. But I knew that in order to propel my business even further that I'd have to expand into new markets--the first being New York. So, I packed up my entire family including my wife and three children, moved across the pond and opened my doors in the middle of New York City in 2012. It was during my first full-day in the office in New York--a market I hardly knew--that I felt I was truly on my way to building something special. If I could uproot my entire life for a company that I believed in, then I knew others would be able to believe in me as well. In just three years, I've grown our US offices from 3 to 25 and am working with many of the most impressive communications talent both in New York and across the entire United States. It was the beginning of a global expansion--we opened our doors in Singapore in 2015 and are rapidly gaining ground within the Asian market." - Jay Haines, Founding Partner of Grace Blue

7. When Apple comes calling

"Mono was probably two years into our business--just 12 people in the office, still a start-up. And we got a call...our receptionist said Rick from Apple was on the phone. Just 'Apple'. In my mind, I'm thinking Apple Vacations, maybe Apple Rental Car? So I pick up the call and ask Rick to tell me a little bit about his business; after a long pause, he says, 'well, we make the iPod...' The story goes on--we had a wonderful, working relationship with Apple. At one point they were our single largest clients and we were handling a ton of their retail work. mono's now grown to over two offices, more than 150 employees, and we've worked with some incredible brands. But that one phone call, that one question and the pause that followed, that was definitely a turning point for us." - Jim Scott, Founder and Managing Partner, Mono