I don't think there is a maximum.  Plug and slot welds are permitted to transfer shear forces only.  They're sometimes used to transmit shear in lap joints, to join components of built up members, or to prevent buckling of lapped parts. Their design and usage is covered in the 2005 AISC Specification Section J2.3., but there's no mention of a maximum spacing.  Spacing beyond the minimum requirements would be a case by case review based on the slot length and width, the weld thickness, and the calculated forces that are to be transmitted through the plane of the faying surface.