What did they know? Not much. And when did they know it?

Only when they decided that truth was less important than starting a war. The New York Times stated it clearly enough below

"The Senate Intelligence Committee report endorsed by Democrats and some Republicans concluded that President Bush and his aides built the public case for war against Iraq by exaggerating available intelligence and by ignoring disagreements among spy agencies about Iraq’s weapons programs and Saddam Hussein’s links to Al Qaeda.
...
The report shows that there was no intelligence to support the two most frightening claims Mr. Bush and his vice president used to sell the war: that Iraq was actively developing nuclear weapons and had longstanding ties to terrorist groups. It seems clear that the president and his team knew that that was not true, or should have known it — if they had not ignored dissenting views and telegraphed what answers they were looking for."

The Senate Intelligence Committee's report, then, shows at best gross negligence by the Bush administration...or at worst, outright lies used with the express intent to start a war that they thought would cement their place in history.

In the second case, Bush and Cheney were right - the Iraq war will cement their place in history: as the worst president and vice president in American history.