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Not sure about the overall length of the game - there are 20 missions, but they are broken down into 1.1, 1.2, 1.3 etc.
I've completed the 'first' mission, with parts numbered from 1.1 to 1.5 - not sure if that counts as 5 missions or 1 mission. If 1.1 to 1.5 are in fact 5 missions, then the game will be very short. If they count as 1 mission, then the game is much longer. Suppose I'll only be able to tell you for sure when I get to mission 4.5 and see if the game ends or not!
But it is a good game.