It was Justice Alito who, at oral argument in King v. Burwell, raised the issue of ruling for the petitioners while possibly adding a stay to the mandate in order to avoid as much dislocation in insurance markets as possible. The six months seems to be driven by the needs of the exchanges (where products to be sold through the exchanges typically need submission and approval well ahead of their sell-by dates) and by the precedent in the Northern Pipeline case.
In Northern Pipeline, the Supreme Court both trimmed the sales of the authority of the federal bankruptcy court system and stayed its mandate for about six months in order to give Congress time to address the inevitable chaos surrounding pending bankruptcy matter and diminished bankruptcy court authority.
The interesting thing about the Northern Pipeline approach is that Congress was required to act fairly speedily, though on a matter overwhelmingly within its control — the appropriate scope of authority of U.S. bankruptcy courts interpreted under the limitations of Article III.
Here, we could toss the matter back to Congress but on a matter not overwhelmingly in their control — whether the states should establish their own exchanges or participate in the federal exchange. The only way Justice Alito's point makes sense is that if the argument that Congress intended the inverse — to require states to establish exchanges.
The ACA represents cooperative federalism — flexible federalism, if you will. Reading the statute as the explicit intent of Congress to, by subterfuge and through a tax provision, usurp the power of the states to decide between a state operated exchange and a federally operated exchange wrenches the statute from its moorings by insisting an inverted reading is a clear reading.