There are several things to check, depending on your configuration.
Deleted items aren't always purged right away. Also, they can be configured to stay until you get a good backup.
Like Zelandakh says, Event 1221 will tell you how much empty space you have in Priv.edb. That is roughly how much space you'll gain on an offline defrag.
There is also an event fired daily after cleaning up deleted items (event 1207). It compares how much deleted items are eligible for cleaning up, and how much was left after cleaning. If you notice that the before/after numbers are identical, check that your backups are working.
If you have to do an offline defrag, it will take up to several hours. There is a way to do it on another server. If you choose that, there is a MS article on how to actually have the other server actually run the defrag. That will be faster if you have a spare server. The speed of the drives is a big factor in how fast it goes. On a slow server it took 4 hours for a 12 Gb database. Ona fast server it took less than one hour.
Read a lot before you do an offline defrag. It's easy to screw it up and potentially lose all your mail. Good luck!
Dan