Concerning the short scenario (10-15 minutes) the designer responded on BGG with:
As for time needed, it varies depending on scenarios. The short ones (D-Day, destruction of Poland & France, etc.) are quite short -- they are "training" scenarios. The full game of WW2 is about 8 hours. Some players say it takes longer; I play a solitaire game in 6-7 hours usually; so that varies.