Halo 3 ODST is being criticized for its campaign length, Halo 3 itself meet the same fate. ODST is unusual as it is an expansion but so large that it can be considered a full game. ODST has other criticisms leveled at it but is the campaign one a far criticism.

So for a FPS what is considered a decent campaign length? In my opinion 6+ would be adequate as the real replay value is typically multi player and this is where the real worth lies. I seem to remember COD 4 had a short campaign as did World at War both two of my favorite games. For me the campaign is almost a fancy tuition phase for both the story and weapons, it is not uncommon for me to quit the campaign early and head straight to multi player.

So the question posed is what do you consider a decent campaign length for a FPS title and why?

I've actually been going over this in my head lately. This is slightly off topic, but I'll try to bring it back around. I have been playing Lost Odyssey, and I am at 38 hours so far. The problem is that I play so little (with the woman and other hobbies, maybe a half hour a day, 2 total on the weekends) that I end up playing the same game for 2+ months (because I like finishing the games I start before doing anything else). Whereas a 9-12 hour campaing can take me a week or two and I get the whole campaign experience, which can be quite a ride (see Gears 2 for example). I actually am okay with a shorter campaign for this reason, and would like to see anywhere from 9-12 hours on average for me, since I'm terrible at FPS this would make a 7-9 for most everyone else.

It depends on which difficulty you play it and if the campaign supports co-op or not.

ODST can be a 9 hours game if you play it on Heroic (the 2nd highest difficulty setting). I played Halo 3 with a friend on Legendary and we needed about 11-12 hours to finish it.
CoD4 on Veteran took me anywhere between 10-15 hours aswell.
Co-op can change the game experience dramatically so it's just like playing a new game.

The thing about shooters is that after awhile they tend to get repetitive. I see a 7-9 hour campaign to be good for shooter. Games like Rage that are going to ship on three discs either need a lot more to do than shooting or will fall prey to being called repetitive.

As others have said, it's more about quality than quantity.  

If you can keep your shooter interesting for 20 hours then sure, give it to me, but I would rather have 6-8 hours of quality rather than 12 of repetitive gameplay.

Just look at some recent games this gen:

• Halo 3/ODST: Short and to the point.  It's not so short you feel completely ripped off and just long enough that it doesn't overstay its welcome.  Would you really enjoy these games if it tacked on another 4 hours of the same thing?
• Uncharted: Platforming and shooting was fun but it was the same thing in the same environments for a good 8-ish hours.  Think if that game were 12 hours, how sick would you be of those jungles and temples?  Just the right length to keep you interested without getting sick of it.
• Bioshock: A good game but also an example of a game that's too long.  The pacing in the game is horrible, the game play was fun but wore out its welcome, and things like the hacking mini games were terrible after the 16th hour.  Don't get me wrong, the story and atmosphere and how those things presented the world to the player is what made them, but the game was too long and overstayed its welcome.

@ twesterm, my 4 playthroughs says that Uncharted doesn't get boring!

Generally, I like 8-10 hours, but I will accept shorter if the game is great... I do feel ripped off though if I am only getting 4 hours of gaming out of a full priced game, or something like that, but most of the time I am satisfied

You can find someone to say that about any game.  There are people that sware that Haze, Lair, and Too Human are all great games and have played through them multiple times. :-p

For the average player, 8 hours is plenty for that game.

I agree with twesterm's summary... for me the best FPS campaign this generation so far was Gears 2... it had good pacing, plenty of epic set-piece battles, and there was a nice variety in environments, like inside the giant worm for example. A 6-8 hour campaign is perfect if it's done right, and it will entice the player to replay it again, probably on a higher difficulty setting.

CoD4 and WaW's campaigns had some nice spots in them, most notably the vehicle levels, but they got very repetitive (and frustrating) after awhile, especially through all the trial and error you endure on the harder difficulty settings. Just trying to make it to that next checkpoint after battling endless waves of enemies with pin-point precision accuracy and an endless supply of frag grenades is an excruciating experience.... AAARRRRRRGGGGHHHHHH!!!!!!

@ twesterm, I know, but there are a surprising number of people here who have played that a game a unusually high number of times, in fact, I will be playing it through again for a 5th time next week

Of course, there are alwyas die hard fans of any game that will keep playing a game until their console wears out only to buy another console so they can play it some more.

It's just most people aren't like that and beyond that most people don't even finish games longer than 12 hours.

Don't worry, I think know Uncharted is a great game.  :-p