ryan mcaffery of ign played the 1st and 3rd levels of halo 4 and it took him 45 mins for the first one and an hour for the third so and average of 50 mins(approximation). so 50 times 8 is 400 mins. which is a bit less than 7 hours. not bad, considering Spartan ops, forge, custom games, and match making! I will edit as soon as I find that link again. it was on ign.