Within a month, I believe.
Just time for them to speed up the plans if they were committed to going forward.
From Daily Times
"Top al Qaeda planners of the Sept. 11 hijackings might have canceled the attack had they known that Zacarias Moussaoui - chosen by Osama bin Laden as one of the pilots - had been arrested, the commission investigating the attacks said on Thursday.
The commission said news of Moussaoui’s Aug. 16, 2001, arrest did not reach bin Laden and top al Qaeda planners like Khalid Sheikh Mohammed before Sept. 11.
“According to (attack coordinator Ramzi bin al-Shaibah), had bin Laden and KSM learned prior to 9/11 that Moussaoui had been detained, they might have canceled the operation,” the report said, using KSM to refer to Mohammed."
The Commission report leaves a lot of questions unanswered. How did they know that OBL and KSM didn't know about Moussaoui's arrest? Do/did they really believe that OSM and KSM had a day to day control over the activities of Atta and his co-conspirators?
According to msnbc in a report titled "Islamic Extremist Learns to Fly," the FBI briefed CIA Director George Tenet on Moussaoui on Aug. 23, 2001, but no information about Moussaoui's arrest was circulated.
It is reasonable to assume that it would have taken some time for Atta to learn of Moussaui's arrest. And as you say that would certainly have forced them to act sooner rather than later - or cancel the operation.
Do you have any references to Beirut story?