Suppose somebody said: "I'll be back in an instant" and then turned up an hour later... You could say : "One hour is NOT an instant"
This is a bit weird with no context but I think it could be used in a situation like this:
Person A: could you please wait for me?
Person B: yes, but just an instant since I'm in a hurry.
Person A: it is fast, just an hour.
Person B: what? An hour is not an instant. Sorry, I'm leaving, bye!!