It always has a very high number at the beginning, but it never takes that long....
However, it can take several hours....I've found this to be true on any device or computer I've downloaded movies or tv shows on - Mac, PC, iPad, Kindle Fire....you name it.
However, even if it does take a long time, that does not prevent you from starting to watch it as soon as it has done a little downloading. Here is the Apple link for typical download times:
Here is a link with Purchased Movie FAQs: