User blog comment:B214/Question and Answers 6/@comment-134.115.166.242-20170221151330/@comment-29963364-20170222035836

It was stated that Rias locked herself in her room for days after she found out that Issei had "died." And the [Bandersnatch and Jabberwocky] were rampaging in the Underworld for days as well.

So Issei was likely dead for close to a week. Give or take 1 or 2 days.