This setting controls the number of days that elapses without a user accessing an Outlook folder before Outlook stops synchronizing the folder with Exchange. For example, say this option is set to 45. User A opens User B's calendar in Outlook, and then does not click on it again for 45 days. Outlook stops synchronizing the data with Exchange and the calendar is no longer up-to-date. The local copy of the data is removed from the OST file. If User A then clicks on the User B calendar 90 days later, Outlook synchronizes the calendar data and starts the clock again for 45 days.