The length of time is probably dependent on the complexity of the book. A member of this forum mentioned that it took about
45 minutes to process about 3,000 books on his SD card whereas I had 1 book that took over 15 minutes to process. Kobo has to create a table of contents for each book and this particular book ended up with a table of contents of over 92 pages in length so I believe that accounted for the long process time.