http://reseller.co.nz/reseller.nsf/opin/227AF9EE93AA37B1CC2573600012C467

Time to dispell those nasty rumours of the game being only 3 hours long.  They started in a thread where a relatively new VG Chartz member said their friend who got the game early beat the single-player game in a very short period of time.

This review is overwhelmingly positive and outlines the only problems as being minor and graphics related. But the focus of the thread is this paragraph.....

 "Keeping Pace

Halo 3 is extremely well paced from beginning to end, introducing new environments, weapons, and equipment at a steady rate. Another testament to Bungie's design chops: the game's intensity and downright awesomeness remains steady for almost the entire campaign. In total, it took me about seven hours to complete Halo 3 on the Heroic difficulty setting, and only 30 minutes of the total playing time felt stale.

Now you're probably saying, "Did you just say Halo 3 only lasts seven hours?" Yes, that's what I said, but there are three things to consider: first, I am way better at Halo than you are. Second, remember that I had two straight days of uninterrputed playtime at the official press review event. And finally, know that even with its short single-player campaign, Halo 3 offers up tremendous replay value thanks to some awesome multiplayer options."

So can we assume it would have taken him 8-10 on legendary, and a much more average player like me 12-15 on legendary?

yes, the reviews so far state 10-15 hours

Do you mean 10-15 hours for the legendary or for lesser difficulties? (please tell me Kain, save me from having to trawl through all those reviews!)

I see no problem. People who buy halo want mostly great multiplayer experience.

anything less than 10 hours is ridiculous, they have been making this game for over 3 years alot of people said bioshock was short like 10 - 15 but im at like over 20 and still have a bit to go. personally anything over 10 and up to 30 is a good length but seriously 10hrs of singleplayer * at least 2 im sure everyone will wanna do 4 co op over live and + 100+ hrs multiplayer..

its 10-15 for normal gamers, they never stated difficulty, im guessing the later of 15 hours is the legendary, either way 10-15 is not bad, and the mult will EASILY make up for the replay value, not including the tons of other features the game has

4 co op on live will have me doing the campaign over and over ill tell you that much

Took me about 9 and a half to beat it on legendary with 3 people. Game is incredible. They finally opened up the matchmaking services and even though there's only 700 people in Team Slayer ranked, its a hell of a good time. They really nailed this one.

Right, it's launched now hasn't it?

no tommorow some got theres early