Ehh... no, that's how far back they look, but only within the last year is sometimes a disqualifier. I can tell you that they will overlook even fairly recent usage sometimes.

CIA: "To be considered suitable for Agency employment, applicants must generally not have used illegal drugs or misused prescription drugs within the last 12 months."

NSA: "Recent and/or frequent use of illegal drugs, including marijuana even in states whose laws permit its recreational or medical use, could raise a security concern, and may be disqualifying"