Tell them that it'll take an hour or so of effort, but they'll have their estimate by tomorrow morning. Then spend some real time documenting the more and less

Tell them that it'll take an hour or so of effort, but they'll have their estimate by tomorrow morning. Then spend some real time documenting the more and less certain areas of change, the likely risks, and the unlikely-but-very-costly risks. The estimate will be dramatically more accurate, but more importantly, they will stop asking for it when it doesn't matter.