According to this entry from the Congressional Record, it only takes 10 seconds to retarget a Russian ICBM.

Mr. Chairman, in testimony before my subcommittee, Bruce Blair, a former targeting officer, said that one can retarget a Russian missile in 10 seconds. Ed Bradley on CBS News, `60 Minutes’ interviewed General Sergev who in fact headed up strategic command and space for Russia and who now is the defense minister.

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