Policy
Keep Search Folders in Exchange online
This policy setting allows you to specify the number of days to keep a Search Folder active when running in online mode. After a Search Folder has not been accessed for the specified number of days, it becomes dormant and no longer remains up-to-date with current contents of folders (viewing the Search Folder makes it active again and restarts the timer). If you enable this policy setting, you may specify the number of days to keep a Search Folder active when running in online mode. If you disable or do not configure this policy setting, then Search Folders always remain dormant.
20077afcc17c L_KeepsearchfoldersinExchangeonline Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\outlook\options\general\searchonlinekeepalivedaysPolicy notes
This policy setting allows you to specify the number of days to keep a Search Folder active when running in online mode. After a Search Folder has not been accessed for the specified number of days, it becomes dormant and no longer remains up-to-date with current contents of folders (viewing the Search Folder makes it active again and restarts the timer). If you enable this policy setting, you may specify the number of days to keep a Search Folder active when running in online mode. If you disable or do not configure this policy setting, then Search Folders always remain dormant.