It varies so widely its often hard to tell. If you get mixed on order e-juice you are looking at anywhere from a week to a month depending on the flavoring of the e-juice. Fruit based flavors, in my experience, need at least 2 weeks or more. Tobacco flavors are sometimes fine at a week, better at 2. Coffee flavors are usually fine with a week while baked goods like cake or cookies and anything cream based need 3-4 weeks. That is just my experience based on my own DIY and what I have found from mixed to order e-juice. Most of the places that mix to order, that I use, date the day of mix and I start my steep count from their date. ANd its just a guideline - sometimes you have to just try a few drips out every week or so until you hit the right amount of time and then remember it the next time you order that flavor for how long it should sit about.
If you order from a custom maker/mix on order site, read the paperwork that comes with it or check their site - they usually have guidelines for how long they think their e-juices should steep for the best flavor possible. WHile longer is usually always better, if you get one you don't like it may never get better for you no matter how long you wait.
As for e-juices from 'manufacturers' that premix and store e-juice before sale, often the shelf life is long enough that they need no steeping. I usually use e-juice I get from china (like Hansen of DK), Italy and even some US companies immediately as i know they have had plenty of time in a dark place (shipping crate) for several weeks while being shipped and sold.