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, Save Unfiled or Save for one of your User Libraries. 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.
Note: most Windows operating systems have a 260-character maximum path length for Windows File Explorer; if you have any issues with accessing files from a back-up, it could potentially be because of the length of your file directory. As a fix you can try renaming the folder name(s) in the file path to reduce characters.