User blog comment:B214/Question and Answers 2/@comment-86.81.216.93-20140529095139/@comment-16281343-20140530085059

It was Volume 7 when he said it was half a year.

There's only one time where an actual date was given, so we only have the events happening around them to gauge the time.


 * In Volume 5, they had a meeting about their training on July, 28.
 * In Volume 7, Issei said he's only been a devil for half a year.
 * In Volume 9, they went on a field trip to Kyoto, and with what I researched that should place the date around October or so. (not sure since I'm not that familiar with the school settings in Japan.)
 * In Volume 18, it's December and Christmas is coming

So, with that said, Issei's been a devil for roughly 10 months.

~We need to have a serious discussion about the time period in DxD. :D