Best way to find out is to make a copy. Use robocopy to copy 4 of the archive folders over to the new drive and time it, then delete them and use the tool in the Archive Configuration utility to actually move them when you are ready. I would guess 300GB should finish in under 4 hours, but it's more about the number of files than the total size so estimates will vary wildly.
Good suggestion, I'll give that a try.
Update: It took about an 1.3 hrs for each 70GB folder.