Push notifications of source control changes can prove occasionally unreliable. It is not unknown for some notifications to fail to be received by Jenkins. Push notification is configured as per the SCM plugin used for each respective branch source.
This trigger allows for a periodic fallback, but when necessary. If no indexing has been performed in the specified interval, then an indexing will be scheduled. If the source control system is not configured for push notification, set a short interval (most people will pick between 15 minutes and 1 hour). If the source control system is configured for push notification, set an interval that corresponds to the maximum acceptable delay in the event of a lost push notification as the last commit of the day (subsequent commits should trigger indexing anyway and result in the commit being picked up, most people will pick between 4 hours and 1 day)