250k Army + 200k Marine Corps + 200k Army Reserve + 360k National Guard = over one million ground pounders and support units. So I give it about a month if we’re kicking it into full gear and take those thousands of armored units sitting in storage sheds out to play.
EDIT: If you mean downsize the entire military (not just the Army) to 250k, then 6-9 months probably, presuming that the draft is instituted for a war that would require a million men.