Charles Carrington in 1929 estimated, from his own experience and that of others in the BEF, that the average Junior Officer in 1916 would spend approximately 30% of the year, ~111 days, in positions "under fire." In that period one would expect on average 4 exposures to direct combat. For him that meant 2 offensive patrols, providing covering fire from the trench lines to support a direct assault, and a single direct assault he himself would take part in.

Source: Dan Todman's The Great War: Myth and Memory