Hi, Dr.Rowlin.
Thank you for visiting Apple Support Communities.
If you are experiencing issues downloading from the Mac App Store, here is a helpful troubleshooting article.
Note: The download time will vary according to the file's size and your Internet connection speed. Refer to this article about iTunes Store purchases for more information about download times for large files.
Troubleshooting the Mac App Store
http://support.apple.com/kb/TS3624
Also, you will have to free up space on your computer to allow the install of Yosemite.
Your Mac also needs:
- OS X Mavericks, Mountain Lion, Lion, or Snow Leopard v10.6.8, already installed
- 2 GB or more of memory
- 8 GB or more of available disk space
How to install OS X Yosemite on your Mac
http://support.apple.com/kb/HT6412
Cheers,
Jason H.