I've found the best time to go to E-mart is after 9:30 pm or so. Why? Well, there are less crowds, which is definitely a plus, but, all the fresh food is on sale for up to half price. Some meat and fish are discounted. Sushi is about 30% off. Ready-to-eat food is usually 50% off or so. Also, sometimes you can get decent deals on vegetables (probably the ones that are about to go bad). Frankly, I still prefer shopping at my market for my vegetables, but sometimes it's convenient while I'm here to pick up something if it's cheap enough.
Anyway, if you're planning a trip to E-mart, I suggest waiting until after 9:30.. they are usually open until 11 or midnight anyway, and check out the sale items.
*Note, the E-mart pictured here is in Daejeon, and is not my photo.