I've seen an estimate that a person may take as much as 20 minutes to get back up to speed after a single interruption. That brings to mind the other subtle ways in which certain events may erode effectiveness.
For example, if the post-interruption time is going to be considered, how about the pre-interruption time? Jill knows that Jack often interrupts her after his 3:00 meeting. As a result, Jill works less effectively after 3:00 - not starting important work and focusing instead on minor items - because of her anticipation of the interruption.
There is a similar effect with memos. Mickey sends a generally-worded memo to his staff that is designed to correct a specific problem caused by Harold. All of the recipients - possibly including Harold - waste time trying to decipher just who and what triggered the memo.
Tally up the effect of both of these practices over the course of a year and the amount of time lost is sizable. So too, of course, is the amount of time gained if the practices are eliminated.