From Teleread:
If you’re going to bundle a code for a free e-book copy with a print book (and charge a little extra for the deal, as with these DVDs), you’d darned well better honor that code for as long as you’re publishing that edition of the book.
[…] e-books aren’t milk. There’s no earthly reason they should have a sell-by date.
While this Teleread article mainly talks about DVDs, the ending has implications for bundling an ebook with a physical book. Specifically, this is the idea that the publisher would offer a book/ebook bundle at a higher price with a limited time offer on the ebook end of the deal. Beyond the idea that there would be a time limit on a redeemable code for the ebook, I can only imagine that there would be an issue with people attempting to re-download the book for whatever reason and being denied the opportunity to do so. While I do like the promise of such a bundle as someone who owns an ereader, there are some outstanding logistical questions that remain to be seen.
Agnostic, Maybe 
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I believe the time requirement would be something similar to the downloadable content codes that you get with video games. The publishers are concerned not with the initial sale of the book, but the resale of the book. If the code were unlimited in its use, that would mean that if a person were to sell the book to a used bookstore and someone bought it, the publisher would get nothing from that sale, and would lose out on the copy of the eBook. I’m not saying that I agree with this, but I can understand their hesitation at having an unlimited code for an ebook.
That also leads me to another thought – piracy. Again, it’s the same problem with video games or just software in general. It’s not difficult to find registration codes for software online, and I can envision a site that would also make available these download codes for eBooks. If the code were unlimited, anyone would be able to use that code to download the book.