User blog comment:B214/Question and Answers 6/@comment-71.15.106.32-20170413060440/@comment-10330822-20170413071659

It probably is only 5 generations back at the most. Remember, Lucifer was around the same time as Zekram, and Vali is his great-grandson.

Purebloods seem to have problems having children, and Sirzechs is already way older than most humans. I'd suspect every generation is a couple millenium apart. The Bible's timeline including AD is like 8,000 to 10,000 years total, so Zekram's probably about that age.