I find 10 or so battles is about enough for modern period campaigns.
A SPWW2 long campaign is an entirely different matter though - as the war progresses you get different toys to play with and the balance between you and the opponent can shift WRT unit effectiveness etc, plus you get to play around in different theatres, geographic areas etc. There 40 to 50 battles will allow you to get used to each change, but limiting the war to say 10 battles would make these "whizz" by..
Andy