Depending on the detection and notification, there can be quite a delay before failover is initiated at all. (I once spoke with a retailer whose primary notification mechanism seemed to be the Marketing VP's wife.)
via Wide Awake Developers Blog by Michael Nygard
I come from a systems management and operations background. On a recent consulting engagement, I asked the tech leads what tools they used to monitor their production systems? "None". Well, I asked, how do you know when there is a production problem? "The user calls us!"