User blog comment:Jopjopjop/DxD-verse Timeline Discussion/@comment-24502975-20141219135656

Volumes 6 - 17 take place between late August and early-mid December. Although there's a bit of inaccuracy between the statement at the beginning of v17 and at v5; IIRC the renovation of the Hyoudou Residence was only a few days before they presented themselves in front of the higher-ups, meaning mid/late July; 4 months later would be late November, and at the end of v17 they talk about how winter vacation is coming up (unless Japan has a different system of winter vacation, I'd assume it's from mid/late December - early January).

But anyway, roughly, you could place 6-9 in September, 10-12 in October, 13-17 in November and early December. I wish I could find that one timeline a guy on Zero's blog had posted.