I have little interest in the Halo franchise, but as a 360 owner I still feel compelled to finish it. I think I got an hour or two in before I quit and would like to finish it, but could not be bothered on my own.

Not long at all with 4 people.  Also depends what difficulty you play it on.  Halo 3 is a fun game, competitive or coop; definitely finish it.

" Not long at all with 4 people.  Also depends what difficulty you play it on.  Halo 3 is a fun game, competitive or coop; definitely finish it. "

Even playing single player, Halo 3 wasn't really that long of a game. During my run (this was on Heroic difficulty) it only took me a day to finish the game (about 6 to 8 hours play time). With four-player coop you could probably get it done in under 5 hours.

" I have little interest in the Halo franchise, but as a 360 owner I still feel compelled to finish it. I think I got an hour or two in before I quit and would like to finish it, but could not be bothered on my own.

If I got some co-op buddies together, how long do you reckon it would take to get through the campaign?

"

If you go fast and don't start team killing... it will probably take you about 6 hours if you don't die often. Its a lot of fun.

Legendary on day one of release it took me and 3 others 7 or so hours.

It's hard to say exactly because you didn't mention what difficulty you'll be playing on, if you have any skulls activated, or if your teammates are any good at the game. Generally though, if you're actually making consistent progress and not just goofing off, it will probably take you less than 6 hours.

about 20 mins

Final driving stage + four player co-op + headsets = absolute hilarity

4-5 hours.

If you play on legendary with all the rediculous skulls activated to ensure you're fucked at every encounter, you could get some serious longevity outta this co op experience!

" If you play on legendary with all the rediculous skulls activated to ensure you're fucked at every encounter, you could get some serious longevity outta this co op experience! "

I think I opted for some harder difficulty? Like one-up from Normal. 

With four people it took me and friends about 8 hours to run through this game on Legendary. We didn't have too much trouble except for driving parts.

if the 4 players are experienced, 4 hours ( probably around 5-6 hours on legendary ) 
  
4 unexperienced players will take you 6-7 hours ( probably around 8-10 hours on legendary )