How many days before vampires take over the world?

A vampire conundrum.

My latest book, Fly-Tipping, was published this week, so to promote it, this week's blog post is inspired by a question posed by one of the minor characters from the book:

"If a vampire drank the blood of one human per night, and every human he drank the blood of turned into a vampire who then subsequently also drank the blood of one human per night, how many days would it be before every person in the world was a vampire?”

Before I answer the question, here's a poll to see how many people get it right. Obviously I can't list every possible answer*, so instead I've grouped the answers into various ranges to make things a little easier.

* Well I suppose I could, but it would be a very long poll!