I've played FO2 for 11.5 years. (Started when I was 9 years old.
)
Played FO1 for around 6 years.
Played FO3 for 800+ hours. (Least favorite as a
Fallout game but as a game on it's own merits it's quite awesome.)
New Vegas 300+ hours. (On hiatus so I don't overplay it before the DLC's come out.)
I don't think there is any other franchise that has kept me playing it for so long.
New Vegas needs some fixing though, it's quest and story is awesome due to it's non-linearity but the game world is so static that everything has become far too predictable for me to enjoy, so I'm on hiatus until DLC's come out or until they add some form of random encounters/leveled enemies into the game.