Which is how long the first two games are. Which is what Bungie has said.N.A said:Between 7-9 hours and I'm playing on normal. I have been exploring quite a bit though.
Edit: As long as you play on the right difficulty and don't rush I'm pretty sure it will take most people 8-12 hours.
Legendary in the first game regularly kicked my ass, though, so that eight hours on Normal and 12 on Heroic turned into 20 pretty easy. Legendary with skull modifiers and co-op should extend Halo 3 in similar fashion.