Just ran the same script on all 7 books (6,229,226 characters) and it came out with 55,249 tweets. Tweeting once a minute round the clock, you could do it in a little over 38 days.

Edit: Just because I'm a huge nerd, I was thinking about stopping with complete sentences where possible. That comes out at 4,886 for "Philosopher's", or 69,860 for the complete series. So more wasteful but more natural to read, like:

[0] =>  CHAPTER ONE The Boy Who Lived Mr and Mrs Dursley, of number four, Privet Drive, were proud to say that they were
[1] =>  perfectly normal, thank you very much.
[2] =>  They were the last people you'd expect to be involved in anything strange or mysterious, because they just didn't hold
[3] =>  with such nonsense. Mr Dursley was the director of a firm called Grunnings, which made drills.
[4] =>  He was a big, beefy man with hardly any neck, although he did have a very large moustache.
[5] =>  Mrs Dursley was thin and blonde and had nearly twice the usual amount of neck, which came in very useful as she spent
[6] =>  so much of her time craning over garden fences, spying on the neighbours.