How should one use Chinese four-character idioms?

Can these idioms be used 'on their own', or must they be used as an inextricable phrase within the context of a complete sentence? This idiom for example: One day, a thousand miles. Can one say that idiom on it's own, independent of a larger sentence? Or must one qualify it with something like, "Today was like 'one day, a thousand miles'"? Your time is much appreciated!