Actually, there's no such thing as octaves - There is only sound :-) But of course we have theoretically agreed to register sound as waves, and divide the waves into pitches. That way it is way more workable for us songwriters.. LOL

But I'd go with Mike, Joe and many others about one octave, unless you are writing for Carrie Underwood or Celine Dion.. But even they do songs within that range :-)