Thanks for using Apple Support Communities.
For more information on this, take a look at:
iTunes Store: Download times will vary
2-hour movie 1.0-1.5 GB 18-24 minutes About 1.5-2 hours n/a2 2-hour HD movie 3.0-4.5 GB 54-72 minutes3 About 4.5-6 hours3 n/a2
Apple TV (3rd generation) - Technical Specifications
A sustained 8-Mbit/s or faster Internet connection is recommended for viewing 1080p high-definition movies and TV shows, 6-Mbit/s or faster for viewing 720p content, and 2.5-Mbit/s or faster for standard-definition content. Internet connection speeds may vary depending on your ISP and other factors.
Best of luck,