For API-650 see 3.1.5.2.b which states that the minimun separation of vertical seams on adjoining shell courses shall be 5t there t is the thickness of the shell plate.
Be careful here. Although the minimum is stated in API 650, for constructability, you would want to have a minimum of 2'. If you don't, especially on thin steel, you will increase your chances of peaking and banding problems in the intersections.