I've not purchased juice from too many different places, so my experience is limited, in my case:
Blue Mist Vaping--needs steeping, the longer the better...I had one flavor from them (rootbeer float) that was vapable right out of the mailbox, but Vanilla Cream and Cafe Carmel needed some serious steepage (is that a word) in fact I've had them for probably close to 2 months now, and just today re-tried the Cafe Carmel and it finally tastes good to me.
EC Blends--excellent right out of the mailbox
Mt. Baker Vapor--excellent right out of the mailbox
DIY Juices I'm having mixed results with, my current Hawaiian Punch was good right away, and remains pretty much unchanged. My watermelon, not so much.
It is hard to wait when a new juice arrives, I always get right into it for a taste.