Creating Encrypted Time Machine disk is taking DAYS!

The reason it is taking long time is that the system is currently in progress encrypting all the pre-existing data on your disk. So the disk is already an "encrypted" disk according to the file system format, and any data you write to it will be automatically encrypted. However there are still remnants of old data on disk that is not encrypted yet, but is in the process of being encrypted.

The problem is that the system treats the whole disk as something that needs to be encrypted, no matter if the remnant data was actually something you used to hold vital data, or it is just never-used or long-gone data areas that hold nothing of interest. It simply has to process it all. The process has a low priority and speed because it usually can take place in the background while you use the computer (and disk) for other stuff.

The quick fix if you haven't got any pre-existing data to save is simply to reformat the drive. When you reformat it, please select it to be Encrypted, Journaled HFS+ in Disk Utility, as the disk will be reformatted as encrypted right away. The old data is not going to be processed, therefore it should be done very quickly.