Some of Halo's flavors are perfect right out of the mail, and some really do require some shelf attention before they're ready for prime time. Give you two examples of opposite ends of that spectrum:
Midnight Apple is awesomepants right out of the mail. Literally. Open up your mailbox, open the package, break the seal on the bottle and drip Midnight Apple into your tank, carto or clearo standing right there with the mailbox still open. Delicious.
Bella Valente, on the other hand, you can put away for 3 weeks or a month, and then it's ready.
On the more moderate middle ground, Tribeca is perfect after a day or two. Prime 15, on the other hand, is perfectly vapable after a day or two, but will continue to mature to get better and better over the course of a week or two.
Here's how you can tell, by the way: You tell by the "nose" -- the smell of the juice.
Midnight Apple? Apple, plain and simple. Always gonna smell like green apple, from the first day to the last.
Tiki Juice? First day you open it, here's what it smells like: Root beer. (Some people say cola.) When it's ready, though, you won't smell root beer, you'll smell spices with a little bit of mint.
Prime 15? First day: Peanut butter cookie. When it's ready, it'll smell like dry tobacco leaves and just a little bit dusty/earthy.
Bella Valente? That one's gonna smell like your mom's perfume on the first day. When it's ready, though, you'll get vanilla and cuban cigar.
These are just my impressions, however. It really depends on your tastes and on which juice you're dealing with.