User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128181036/@comment-1359322-20140131174448

Issei will enter his third year as a high school student in Volume 19, it can be pretty much assume that Rias and Akeno will graduate on that volume or Volume 20. Ishibumi already confirmed another theme of the 4th story arc is "Generation Change". The generation change here refers to Rias and Akeno leaving the club to Issei and the others.