I knew at once he wasn't joking. Even he wouldn't joke about such a thing.
And yet, for a long moment, I didn't react at all.

Или такая:
The others were more surprised than I was. For a long moment no one made any response.

Просто "некоторое время"? Хотелось бы подчеркнуть длительность момента, но как - не знаю.