However long it takes.
If you request e-books like Harry Potter, Ray Bradbury, or To Kill a Mockingbird, you're going to have a very long wait because the authors adamantly refuse to allow their works to be published in e-book form.
If the paper book publishing contract has expired, you're going to have to wait until the author or the author's estate decides to work up an e-book publishing contract with someone. Also you're going to have to wait until they either type in or scan in the original book, then proofread and correct the mistakes, and come up with cover art and other supporting material.
If it's a book that the publisher hasn't gotten around to putting out in e-book form, you'll have to wait until the publisher gets around to it.
If it's an e-book that Amazon has an exclusive distribution deal on, you'll have to wait until the deal expires.
If it's an e-book from a publisher that B&N doesn't have a distribution contract with, you'll have to wait until a distribution contract is negotiated or until PubIt! is running and the publisher decides to use that.
If it's an e-book that was simply overlooked, it could be just a few days. Or not.
