From the information that I have been able to gather in my study of the the Great War, most soldiers were there for the long haul. Those that went home during the war were primarily those who were on leave, or were so seriously wounded that they were no further use to the military. There are two primary source material that you may want to look at: All Quiet on the Western Front and [Cannon Fodder: An Infantryman's Life on the Western Front, 1914-18](Cannon Fodder: An Infantryman's Life on the Western Front, 1914-18). They are a good resource for a first hand account on what conditions were like during the war.