User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170307010853/@comment-87.9.173.147-20170308171120

Less than 20 years... Considering that Kurenai and Airi seem more or less 17-18, a little more than 10 years I guess. Well the reincarnated devils are easier to get pregnant thatn pure-blood devils, obviously, but it's clear that Issei really "worked" with abnegation to get all of them pregnant (especially Rias and Ravel). By the way, if I remember well. The first time Xenovia tries to seduce Issei she tells him that if they have sex every day he should have been able to get her pregnant in within 10 years, but if they would have done it 3 times a day it would have been possible to obtain that result within 5 years. So you can more or less imagine how much he had sex with each one of them. lol