Autoshrink will shrink the database when there's 25% space available, per http://msdn.microsoft.com/en-us/library/bb522682.aspx
But it's almost always a really, really, really bad idea to turn on autoshrink.
You can also schedule a database/file shrink to run at a regular interval through an SQL Agent job. Very easy to do. Details here: http://msdn.microsoft.com/en-us/library/ms189080.aspx
Again, this is a really bad idea and I recommend that you not do it.
Shrinking a database should be done only when it absolutely has to. It has significant impact on performance issues (index fragmentation), file level issues (physical fragmentation), can have a negative effect on backup/recovery plans, and a few other issues.
I can't stress too much that automatically shrinking a database, either through a scheduled job or through autoshrink, is really, really bad.
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