how long would it take to beat halo 3 with 2 players?

[Heather]

i like to play halo 3 on xbx live with my friend but she sucks, so we're gonna play the whole game (i haven't even done that yet) to give her some skills. how long will it take? i have past experience beating halo and halo 2 on legendary LOL

 

[nicolas]

for me there's a level that was like REALLY hard and it took me a long time to finish it it had TONS of flood forgot wat it was but if u finished on ledgendary with 2 players youll have no probolem

 

[the guy down the street]

on legendery i beat it solo in like 12 hours but i am a 43 in live so i am very experinced so i woul;d think somewhere in between 10 to 15 hours is my guess and that was the first thing i did when i got halo 3 and now i am a level 43 so she will diffinitley improve her skills at halo 3