Backing up Libraries within Imaging is a very simple process. It does require that you download Retriever for Imaging, which is the free tool that you'll need to perform the backup. After downloading Retriever for Imaging, it's simply a matter of clicking the Library menu within Retriever and then selecting Save Client, Save Corporate or Save Unfiled. This will allow you to select a location on your computer where you would like to save the Library and then to begin the download process. The amount of time this will take just depends upon the size of the Library you're saving and your download speeds.
For more information on downloading and using Retriever for Imaging, see this post.