For my smaller OST data files, it completes in just a few minutes. But on my larger OST mailbox files that are over a Gig in size, it typically takes 20 to 30 minutes to complete. I have even seen situations when the compact can take up to an hour to finish on huge data files, and sometimes even longer.

What does compacting an OST file do?

Compacting Outlook data files reclaims the space that is unavailable because it was previously occupied by an item. This can greatly reduce the amount of space that the OST or PST file occupies on your hard drive, and helps Outlook function efficiently.

Can an OST file be compacted?

To reduce the size of your Outlook Data File (. pst) or Offline Outlook Data File (. ost), you can manually compact the data file.