First thing you do when you get a new juice, take off the lid and let it rest for a few days. You will see a change in juice color usually, but not always. Once it has sat for 3 to 7 days just cap it up and give it a really good shake, until you see tiny bubbles through out the juice, then take cap and give it a good smell. It should have better aroma and be a little more complex tasting. Not every juice will benefit from long term steeping, but all will from short term. This can cut down on the chemical or artificial harshness of some flavorings as well.
If in doubt of steeping time just ask the vendor and they will tell you how long to steep their juices.