RPGs and RTS game campaigns are usually pretty decent lenth, but FPS game campaigns are much too short. The last FPS game that I played that had a decent lenth to it was "The Suffering", and its a few years old.
I think 15-20 hour single player campaign lenth would be great for FPS games, but sadly they always have the shortest lenths. I usually don't play through the entire campaign in RTS games, and most RPG games get boring long before I've beaten them.
My attention span is longest for FPS games, theres only two FPS games that I didn't beat; "Will Rock" and "The Fortress of Doctor Radiaki".
"Will Rock" was awful. They took all the worst elements of FPS games and packed it into one big heap.
And the Radiaki one, I was really young when I had it, and all it was was a knock-off of Wolfenstein. There was no competition there as to which one to play.
Edit: Got all nostalgic on Radiaki,
looked it up here. It was out in '94, later than I thought. I guess I had Doom to play instead, not Wolf. Mobygames score: 1.5/5 , ouch.