As of 2014, the video game that had the longest development time was Duke Nukem Forever, which is followed by Diablo III. These two games took 15 and 11 years, respectively, between announcement of development until it was released to the market.
Duke Nukem Forever was announced to be in development by 3D Realms in 1997. It took the studio eight years to release any game footage before 3D Realms ended up being downsized. The game was eventually finished and released in 2011 by 2K Games. Diablo III began development in 2001 but was not formally announced by Blizzard until 2008 at the Blizzard Worldwide Invitational. The game was not released by Blizzard until 2012, four years after its initial announcement.