Depends upon the juice vendor and the individual juice flavor.
Some juices are the pull off the shelf variety from the store room and don't require any steeping whatsoever.
The juices from ejuice vendors that mix each individual order may require anywhere from 4-14 days to steep. I have found the complex flavors like coffee and bakery flavors to require longer steep times. Most of these type of vendors will provide suggested steep times for their juices somewhere on their website or sub-forum if available, if not an email inquiry will provide this information.
Steeping might include merely letting the juice rest in a dark cool place. Some people like to shake the bottle, let open the cap for a day or two, return the cap and shake again. It's all quite subjective and individualized. What ever you find works best for you.