Policy
Keep Search Folders offline
This policy setting allows you to specify the number of days to keep a Search Folder active when running in offline or cached 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 offline mode. If you disable or do not configure this policy setting, then Search Folders always remain dormant.
490dbae26601 L_Keepsearchfoldersoffline Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\outlook\options\general\searchofflinekeepalivedaysPolicy notes
This policy setting allows you to specify the number of days to keep a Search Folder active when running in offline or cached 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 offline mode. If you disable or do not configure this policy setting, then Search Folders always remain dormant.