Before Dave Walsh began playing video games professionally in 2004, he crunched the numbers and calculated what he could realistically make on the competition circuit. He showed the math to his parents, and they gave him their blessing to leave a summer job at the post office.

At his first tournament that summer, Walsh won $5,000, slightly more than he would have made all season at the post office. Two years later, he signed a three-year contract with tournament operator MLG—Major League Gaming—worth $250,000. Six years after that, he had retired from pro gaming.

Video game enthusiasts play "League of Legends." Jean Chung | Bloomberg | Getty Images

Walsh's story is not uncommon. Roughly a decade since competitive gaming—or e-sports—became big business, many top gamers have hung up their control pads well before they hit 30. The few gamers who achieve fame and fortune work day and night to maintain their status.

(Read more: Watching other people play video games...that's e-sports) "You need to dedicate pretty much your whole life to it, and there's so much competition now it's really hard to have a balanced life and be a pro gamer," said George "HotShotGG" Georgallidis, a retired e-sports competitor and owner of the team Counter Logic Gaming. Competition is so intense in part because riches remain elusive to most players despite the massive growth in e-sports viewership and tournament prize pools.

Last year, tournaments awarded more than $15 million around the world, up from just over a $1 million a decade ago, according toesportsearnings.com. Still, while 27 million people play "League of Legends" each day, just 40 professionals earn salaried positions in the North American League.

(Read more: Nintendo mobile games could draw new players) Today, players can also earn ad dollars when fans livestream their games on websites like Twitch and Azubu TV. But gamers say only a handful of players can earn enough to make a living. Things were different when the 29-year-old Walsh—known as Walshy among gamers—began playing "Halo," the best-selling shooter game published by Microsoft. Competitions were few and prizes were modest. But during Walsh's college years, organizations like MLG professionalized tournaments, drawing in not just more competitors, but spectators, too. That was when Walsh decided to leave college and compete full time.

"Around the end of 2005 the tournament scene started getting bigger and bigger, and I realized it was a once-in-a-lifetime opportunity and I could always go back to school."

The same year Walsh signed with MLG, the company began broadcasting matches on USA Network. The show increased his celebrity, and he attracted sponsors such as Red Bull, Old Spice and Sony Ericsson. Walsh won't say how much those deals were worth, but sponsorship earnings eventually eclipsed his tournament winnings. At his peak, he said he was making low six-figures.

A decade into his career, Walsh had been top-rated in five games. Still, he knew a competitive career wouldn't provide a living for much longer. The "Halo" trilogy and other shooting games that he specialized in were losing clout on the competitive circuit. Transitioning to another genre would mean starting at square one. Even if "Halo' remained a top game, Walsh wasn't sure he had it in him to continue playing at the same level.

"A lot of it is drive and motivational," he said. "Being at the top for a long time, it's hard for me to practice as hard as this 16-year-old kid who wants to be better."

Fatigue is a very real aspect of gaming, say professionals. Gamers are not playing "Super Mario Brothers." They compete in complex games that require elaborate strategies and problem solving in real time during grueling hours-long tournaments. Walsh compared tournaments to taking a long, drawn-out test.

Georgallidis started playing "League of Legends" in 2011, a strategy and battle game developed by Riot Games, and quickly became one of its stars. He was one of the first players to attract tens of thousands of streaming viewers to his online channel. At the height of his career, he said, he could earn upwards of $2,000 in a day from ad revenue.

(Read more: Videogame star aims to make gamers heroes)

To maintain his viewership and standing—and the income they generated—Georgallidis played "League of Legends" for about 14 hours per day. That allowed him to create Counter Logic Gaming and fund the team's travel and operating expenses, but he couldn't keep practicing so much while fostering his business and maintaining a personal life. Last year, he gave up his spot on the team to focus on coaching and managing the players.

Along with fatigue comes stress. Players in Riot's professional leagues scrimmage for about eight hours a day against teams they'll eventually face in high stakes competitions. For much of the rest of the day, they're running drills.

"It's a dream that everyone fights so hard for, and I only see it getting more and more competitive and more and more cut throat," Georgallidis said.

For these reasons, gamers need to think about other options by their mid-20s, said Tobias Sherman, co-founder of eSports Management Group, a sports agency for gamers. That's why he speaks candidly with players about how long they can reasonably compete and helps them plot out their post-gaming lives.

Nick “Tasteless” Plott