[How could they have known that ahead of time unless al Qaeda call and tell the DOD that the attack was going to last a certain number of hours?]
It's like a fire department not responding to a house fire because they believe the house will be consumed before they can get there.
as a volunteer fireman, we responded to a trailer house fire that was 30 miles away. it took us about forty minutes to arrive on at the fire. anyone knowing anything about fire and trailer houses knows they go up quickly. we got there in time to save the house.
the same is true with bengazi, the effort needed to be made. even if they came late they could have enacted a terrible price for eFing with us