Going by the cycles of the clock spoken of in game, I figured that the entire game could have taken place over the course of two weeks. A very hectic two weeks, mind you, but it could come in at two weeks.
I think it probably took 3 months (FS1 is actually closer to 2 months than three). That would keep it around the time span of 18 months since the start of the confilict for the bulk of it (17, 18, and 19 months could still be refered to as 18 months without it being too off). After the rebellion is finally crushed, then all bets are off on the lenght.