Correction: they had plans to hijack airliners and crash them into American buildings. How long do you figure it took them to realize that this plan might be suitable for their #1 target, the World Trade Center?
The hijackers only entered the US months before the attacks -- they weren't sleepers, the plan wasn't in place years before.
So based on the fact that 13 of the 19 hijackers entered the country between April, 2001 and June, 2001, you were able to nail down when this plan was put into place?
Yes
The original plans were to fly a plane into the CIA HQ. After we nabbed Yousef in 95, he was flown through NYC in a helicopter... He stated that they would be back to bring the towers down, and that his arrest couldn't stop the plan. I think that it is quite likely they planned on using planes to take down the WTC at some point around, or just after then. KSM was also noted to have mentioned this possibility in 1998.