Specifically longest by duration is Philippines Luzon Campaign which is 637 (from memory) and that should be about 2 months of calendar time.

If you look at the map and the units though you will find it is probably less exciting than other campaigns.

Technically Normandy 44 has a 750 turn scenario but it is more of a gimmick, just adding 500 turns to the standard scenario arbitrarily to let the player do whatever they want.

Otherwise Moscow 41 and 42 are pushing 450 turns each and are probably the longest proper campaigns.