zmprov ma email@example.com zimbraPrefOutOfOfficeCacheDuration 1dduration syntax:^\d+[hmsd]?$.
If [hmsd] is not specified, the default is milliseconds.
(you could also modify it on the COS)
Thus duplicate away messages will not be sent to the same user in the given interval of 1 day. On day 2, if they send that account an email, they would again get back one notification, then no notifications for another day.
Setting to 0 would send an out-of-office notification in response to every email received. (you want this-but it's generally not a good idea, as your sending responses to spam etc...
+ If you have user A with an away message up, user B sends an email, and immediately put's up their away message (with the attribute set to 0) you could technically get a loop.
Therefore 1d and up is probably a better setting to use.
I prefer 3d (or every week 5/7d depending upon how long the user is gone) ie: If their gone a month-then every week, (this is what I set the COS to) unless they specifically ask for 3 days.
You also might be interested in:
Search bugzilla for the requests to add these via admin/user settings...