How long is a piece of string?
This is really a tough (and maybe impossible) question to answer. A site like BookPool is much bigger than you can see from the out-side. It probably has connections to financial systems, delivery systems, stock keeping systems and other back-end databases that take care of the administrative organization.
Even if you are "a good .NET Developer", I don't think you could build a site like this on your own. You'll need database specialists, graphic designers, usability experts, work-flow designers, etc etc. So, a site like this is usually build by a team of specialist. My rough guess is that it will take months and months of work (and probably more). BookPool probably invested more time than that, because even after the initial release of the site, I am sure they added new features to keep up with Amazon ;)
If you're new to programming, a would consider a less ambitious project to start with. This allows you to focus on the different technologies you need to be acquainted with, before you "drown" in the complexity of such large projects.
Everyone is unique, except for me.