User blog comment:B214/Question and Answers/@comment-50.11.177.49-20130529004308/@comment-9297146-20130529022422

From what I've gathered the current Great Maou did participate in the great war. The war ended at least 300-500 years ago* since some of the oldest servant devils, which were recruited because the demon populations were so low after the conflict, are around that age. Sirzechs and Ajuka were known as the Ace's of the demon side so it's safe to assume that the current leaders of the demons participated in the war. Afterward the civil war between devils factions had Sirzechs and Grayfia fight and fall in love getting married afterward.


 * Ok now this is an interesting way I use to figure out just how long ago the great war ended, using the amount of "Senpai" inside of Issei's Sacred Gear. There were at least 20 senpai inside of his Sacred Gear, a conservative estimate, the battle where they, Albion and Ddraig, were sealed killed God and the Great Maous causing an abrupt end to the war. The age estimate of the senpai range from early teens to late middle age so we'll use a median of 25 years of age. Right after a user of either Sacred gear is killed they are automatically transferred to another user.

So,

20 senpais x 25 years of age each = Minimum 500 years ago. Now this is just an estimate and can go as low as 300 to as high as 1000 years ago. But it's just food for thought.