According to the Kindle manual, the only commercial audiobook format supported is the one from Audible. However, it also says it can play MP3 files natively off an SD card, so I can't think of any reason why you couldn't record an audiobook into MP3 format, stick that sucker on an SD card and listen to it from the Kindle.
It seems to me that the only limitation on the length of the MP3 file would be the size of your SD card. I wonder though---presumably your idea is to create an audiobook that would be made available for Kindle owners to purchase? I mean, I can't imagine you'd record yourself reading a book aloud just for your own enjoyment later. Since Amazon has married the Kindle to Audible for audiobook content, I'm not sure where you'd find an outlet to sell your own, MP3 audiobooks. Interesting idea, though.
