Severus looks amused - darkly, the only way that he does - but says nothing. He's not going to just lay out his loyalties for the other man, no matter what their relationship should or shouldn't be. If he knows Severus has the capacity to turn spy, then he knows Severus isn't someone who's terribly worried about being caught by a far more deadly authority. He hasn't got much to fear beyond harassment from old colleagues.

"Long enough to be bored."

Yes, Narcissa is going to kill him, he knows.