I address the problem by giving a range. Giving a single number is the real problem. For example I might estimate something as two weeks plus/minus six weeks,

I address the problem by giving a range. Giving a single number is the real problem. For example I might estimate something as two weeks plus/minus six weeks, or a month plus/minus a week. That range is because of the usual factors (scope creep, things that have never been combined before, testing, meetings, holidays etc). Sometimes the questioner picks up on the two weeks minus six weeks is a negative number. I then explain that existing functionality could be used, goals could be achieved in other ways, or maybe it just isn't really needed.