That's a good point. IN the case of Spain, the country was already riddled with AQ cells, so this may have made it easier for them.
In addition, I'm sure they're going to find connections with the domestic terrorist movement, ETA. The blasting caps were manufactured in Pais Vasco, and the Algerian arrested today was caught in San Sebastian, a Basque city, where he was involved in the drug trade and upon a drug arrest in January had threatened that hundreds of Spaniards were going to be killed in Madrid. ETA controls Pais Vasco very closely, and anybody who is involved in either the drug trade or terrorism up there certainly couldn't do it without ETA's consent. So I think having the existence of a home-grown terrorist network also contributed. (I still think OKC was a joint AQ-domestic operation, of course.)
I have never thought it took them years, in any case. They seem to have an "on demand" type of terrorism: lay the groundwork at their leisure, and then when the moment is right, strike. It just hasn't been that advantageous to them before to strike quickly. I think, alas, we're going to see a series of these strikes now. Just my paranoid opinion.
If you Google the word "Fatwa" and read their orders, you will see that in 1998 they decreed that all Americans should be killed. This message was far before 9/11 and the War in Iraq. Your observation about the time element is seemingly quite accurate. The negative, however, is that they've had lots of time for preparation.
23 February 1998
Shaykh Usamah Bin-Muhammad Bin-Ladin
Ayman al-Zawahiri
The ruling to kill the Americans and their allies -- civilians and military -- is an individual duty for every Muslim who can do it in any country in which it is possible to do it, in order to liberate the al-Aqsa Mosque and the holy mosque [Mecca] from their grip, and in order for their armies to move out of all the lands of Islam, defeated and unable to threaten any Muslim. This is in accordance with the words of Almighty Allah, "and fight the pagans all together as they fight you all together," and "fight them until there is no more tumult or oppression, and there prevail justice and faith in Allah."
We -- with Allah's help -- call on every Muslim who believes in Allah and wishes to be rewarded to comply with Allah's order to kill the Americans and plunder their money wherever and whenever they find it. We also call on Muslim ulema, leaders, youths, and soldiers to launch the raid on Satan's U.S. troops and the devil's supporters allying with them, and to displace those who are behind them so that they may learn a lesson.