Outlook will compact the pst automatically when the amount of "white space" (space used by deleted items) exceeds 20% and it has some idle time. Outlook will work on compacting the pst when it and other programs aren't using all of your computer's resources and in most cases, it will recover the space within minutes, often starting as soon as you emptied the Deleted Items folder. However, if you deleted a large message (many MB in size), it could take hours or days to recover all of the file space no longer used by the message.
If you want to reclaim the space immediately, you can run Compact manually. Under normal circumstances, it will take several seconds to several minutes to complete the compact process but if a single message was huge (multiple MB), Outlook may seem to stop responding when it is compacting the space used by that message.
To compact manually in any version of Outlook:
- Right-click on the root folder of the pst file and choose Properties.
- Click the Advanced button to open the Outlook Data File dialog.
- Click the Compact Now button.