People will ask a question like "How long does it take to plow a field?" and get useless responses like "How long is a piece of string?", the implication being

People will ask a question like "How long does it take to plow a field?" and get useless responses like "How long is a piece of string?", the implication being that no satisfactory answer could be given because a specific answer is dependent on factors not known in the question. This is done in lieu of providing a useful order-of-magnitude answer. How long is a piece of string? To be useful, at least a few inches. A common task, sewing a button, needs about 12". Most string in a store is 200 yards long, but this is enough to do many things. String longer than that is very rare.