SQL Server automatic transaction log truncation
No physical truncation actually occurs, the size of the file would not get smaller. 

Parts of the logfile (virtual logfile/vlf) are merely flagged as 'the content is allowed to be overwritten'. If such an event exists, and I know of none, there would be no practical duration.

If you are referring to 'auto-shrink', I would highly recommend you read this post 'Auto-shrink – turn it OFF!' by Paul Randal.