I'd guess that figure's went up as understanding of the condition increased, but that may be skewed stats as obviously there were the crises in 1918 that needed anyone on the line.

Alexander Watson puts the numbers at c. 5% of total casualties and c. 10% of casualties in major battles, for the British Expeditionary Force and the German Army on the Western Front. His book Enduring the Great War is an excellent read for anyone interested in how soldiers in the aforementioned armies managed to survive mentally. A caveat should be added, however, that some cases may not have been recorded as 'Shell Shock', but 5% at least for the British and German armies seems to be the number.