I would say an 8 year gap between attacks on the WTC shows they can restrain themselves. And wait thing out till the timing is to their best advantage.
You might be surprised to learn that they didn't sit on their hands for those 8 years, they hit a lot of other targets.
pg. 95, 'Losing bin Laden'
"Recruits are divided into small groups and urged to shout ayahs (verses) of the Koran in unison. Early in his training, Rasheed Daoud al-Owhali recalls repeatedly chanting this Koranic verse: 'I will be patient until patience is worn out from patienct.' Nothing better summarizes al Qaeda's methods."