User blog comment:B214/Question and Answers 4/@comment-201.141.85.213-20150102101912/@comment-25935458-20150102135529

In volume 14 it was stated that Issei's life force "becomes zero or turns into infinite" since he was revived by Ophis and Great Red. Azazel stated  "I do not know what is going on at all"

Ishibumi has not given us any more information on Issei's life force state up to this point. Hopefully he will include something about it in future volumes. Until then your guess is as good as mine.