Confusing question, you start asking how long to install then asking why so long to download, these are two entirely different operations.
How long to install will depend on your computer's capability and the type of instalation, clean or upgrade?
Downloading is a process that downloads the instalation files from the Microsoft server, this depends on your internet connection provided by your ISP.
An upgrade can take a lot longer then a clean install, 3 hours with 750 gb used doesn't sound too bad, good thing you have a fast cpu.
How does 20 hours 14 minutes grab you?
http://blogs.zdnet.com/Bott/?p=1299