Team Fortress 2 started development in 1998, and wasn't released until 2007 (a total of 9 years).
Duke Nukem Forever started development in 1996, and wasn't released until 2011 (a total of 15 years).
The important thing about both these games though is that they went through multiple iterations and multiple engines before finally being finished. For example, many people complain about Duke Nukem Forever for it's quality not living up to fifteen years of development, but that's because the DNF you see today actually took nowhere near fifteen years to make. Rather, DNF as it is was probably in development since ~2008 or so. Any versions of the game before that were totally scrapped and lost in favor of newer technology.