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I mean, while every week means one historical month has passed, this has almost no influence (for most of the campaign) on the time you have to finish your games.
Apart from the armistice date (18 September) you are not constrained to finish the games in the same month they started. So for example, games created on January 1917 can last, if needed, up to 23 full weeks
Obviously games created later have less time, but even those created on September 1918 (when Sopwith Snipes come out) have still good 3 weeks to be played.
Indeed larger Snipe scenarios might have trouble finishing before the armistice, but that would also some way reflect the difficulty of making such late new plane introduction have effect on the outcome of the war.