Why it takes 19 days to erase a Time Capsule harddisk?
The time needed to erase a hard disk depends on the size of the disk, the erasure method (1-pass/3-pass/7-pass) and the number of bad blocks.

If your drive contains bad blocks and you try to erase them, the drive may try numerous times to reread/rewrite the entire block and often invokes the error correction to rebuild the data depending on the erasure method.

Examples:  


4 weeks ago i had to erase a 320 GB HDD with (finally) 23 bad blocks. It took about 4 hours to finish the task (1-pass).  
6 month ago i examined a breaking HDD. Every single bad block took 5-10 minutes to be validated.


Furthermore the algorithm estimating the completion time of the erasure will be misguided depending on the position of the bad blocks.