@Tamerlane Per the chapter she first appears in, the office lady met Nazuna approximately six months earlier. If we assume that Nazuna bit her that time, then she has at most six months to fall in love with a vampire- and since she's less likely to fall for Nazuna, her only hope is to fall for Kou
unless the time limit is specific to each vampire; i.e. for Nazuna it would be six months from now, for Kou it would be one year from the first time he bit her (and all this assumes that Kou even succeeds in becoming a vampire.)
This is not how i am currently thinking it works, so i am not optimistic. Six months is a pretty short time for things to start happening in manga, unless A) this is going to be a short manga, or B) this manga's focus will shift if/when Kou becomes a vampire, but the story will keep going.