March 2003:
A senior administration official who briefed reporters Monday on condition of anonymity said Rumsfeld "has right along said that he thought that fighting was likely to last weeks, not months." Rumsfeld told troops last month that "it could last, you know, six days, six weeks. I doubt six months." Rumsfeld also contradicted the Army chief of staff, who told the Senate that "several hundred thousand" troops would be needed to occupy Iraq. "Far off the mark," Rumsfeld said.
Jan 2004:
Vice President Dick Cheney warned that the battle against terrorism – like the Cold War – could last generations, according to a report in the San Francisco Chronicle.
...
While polls show that many Americans support the president's aggressive war on terrorism, an expert at the U.S. Army War College, Jeffrey Record, recently released a report that concluded the war in Iraq might have set back American efforts to stop terrorists by diverting precious resources to a battle that will do little to prevent new attacks.
Record concluded that the war on terrorism "lacks strategic clarity, embraces unrealistic objectives and may not be sustainable over the long haul."
I think it is clear that nobody knows.