Hi. Is there any way you can see How much files is left to Sync\Index in the admin console of DSM… Here…
So unfortunately you cannot really tell how long it is going to take. You can get a feel from looking at the progress bar, but its based on a bunch of factors, so you might not get a really good estimate
If it means anything I have 12 hard drive with 30tb that took about two days, and the was before I cleaned it (clearing out all the deleted files) which now bought it down to 12tb.
My own personal photo collection with videos is about 300,000 items plus other people.
It took three days went I swapped out a drive.