The Halo games side + main story were all around one of these categories. What's your guess?
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Are you talking about completing each mission on both difficulties solo? If so that may add a decent amount of time to it. Personally I think the main story will be beaten very quickly, at least on lowest difficulty with a fireteam. As for side missions go, they are infinite & random so may take long time to get each mission. Overall I think it's safe to figure 10+ hours solo & probably 4-5 with a team