The most often used expression is "Time flies." Example: "How time flies. Before we know it, it's Christmas again."
I think for a learner,
1. For "oral production", the pragmatic thing to do is to learn the standard expressions and stick to them until he gets to an advanced level. Then he can learn and speak a greater variety of expressions.
2. For writing, he should also learn and stick to the standard expressions first, and aim at error-free, simple and precise writing. This is the best writing even for native speakers.
3. For written comprehension, he should understand a wide range of expressions, but there is absolutely no need to imitate different styles when he writes or to introduce extraordinary or colourful expressions into his own writing.
The most successful learners are the diligent and PRAGMATIC learners. Pragmatic learners stick to simple standard expressions.