If there is no Wi-Fi connection or the connection is considered "too slow", it will not download the build.
If you are connected to a fast Wi-Fi, the phone will "periodically" check to see if downloading is possible, and will automatically try again. It will also wait until the phone isn't being used for a bit, so you'll need to probably not be playing games and such, or browsing, etc. while waiting for it to download. Mine took 10 minutes of inactivity and high-speed Wi-Fi before it would begin downloading. Your mileage may vary.