There's a mild twitch in Severus's expression which might be him biting back a second flash of amusement. He's not certain whether Lucius means 'for a time' because she's well on her way to being a married woman (unlikely), because Draco will come (more likely), or because the first war is lying in wait like a trap for her (that's probably it).

But the very idea that Lucius would ever resort to such crude humour as the first idea sets him not-quite-laughing again.

It's becoming clear to Severus that he is at that point for a person when frustration is so overwhelming, laughter is the only recourse. Helpless laughter, followed by irrational anger if emotions aren't curbed.

He refreshes his wine from the bottle between them, his jaw tightly clenched. When he does speak, when he has regained enough of his sense of emotional control, a few seconds have passed. Enough to nearly become an awkward silence. "I should hope she wasn't venturing into the fog. Not that I find the endeavor itself deserving of disapproval, but Narcissa at eighteen never struck me as the sort one might wish to take on an expedition to Muggle London, much less into the dangers surrounding the city. Narcissa at thirty, perhaps."