We cannot answer that question, because it depends 100% of your environment.
If you have tapes, worm, etc... then it might takes days/weeks/month... And if you don't have enough tapes drive, maybe even years!
If everything is on disk, then it depends on where the document are stored -> network speed, network congestion, disk speed, if the servers are idle or not...
My advice would be to extract a sample of documents... and try to do some math / estimates on how much time it will last...