User blog comment:B214/Question and Answers 4/@comment-24331706-20150319150433/@comment-24098614-20150320084516

The lifespan of Issei right now is unstable because after he unleashed the Juggernaut Drive to save Asia from Astoroth it reduced his lifespan to a 100 Human years because it is nearly 90 percent of his lifespan, and had to receive Senjutsu treatments to Koneko to add to his lifespan because generally Devils live way over 10,000 years in average.

The death of Issei in Volume 12 resulted in giving him a new body with the flesh of Great Red, and his lifespan borders on both infinity or having zero so he still continues to take Senjutsu treatments because his lifespan is still unstable even with his new body.