Indices in VSys should be updated every few days. This notification is generated if they have not been run recently, but only if the subscriber hasn't been notified within the Don't notify about this time period. Unlike object-specific notifications VSys will re-notify you about this condition more than once, hence the Don't notify about this... setting to prevent a constant stream of duplicate notifications.
If multiple indices need to be updated, only a single notification is generated - not one per index.