Chris Butcher, master of Halo AI, posted a message in HBO's forum about Halo's difficulty levels. Here's a snip:

for an epic battle (or if we're playing co-op) I pick Legendary. it took me about 60 hours of continuous play to finish the game on this difficulty, and that's after months of developing and playtesting the game. it's a very different kind of challenge to, say, myth's legendary difficulty. many of your enemies will be outflanking you and firing from multiple directions, and they'll retreat and build up their shields if you damage any individual too heavily. also, if you don't know what you're doing, or you get taken unaware, it's very possible for a single covenant elite warrior to nail you (and don't talk to me about the commanders).

There is also a follow-up post worth reading that further comments on the time-to-complete issue.