Pokémon Red and Green reportedly took six years to complete.
The development period was unusually long for a Game Boy project. Satoshi Tajiri and the Game Freak team had to build a new role-playing game centered on collecting, training, battling, and trading creatures through the Game Link Cable.
The project began after Tajiri conceived the Pokémon idea around 1990. Development placed heavy financial pressure on Game Freak; the company reportedly struggled to pay employees, and Tajiri did not take a salary during the process.
The finished games were finally released in Japan on February 27, 1996. Their success helped revive interest in the aging Game Boy and led to a much larger multimedia franchise. The six-year figure refers to development of the original games, not the entire Pokémon franchise.