I agree with Rat, Don't leave the cap off!
First, smell your juice, by squeezing the bottle (carefully) and smelling what comes out. If it smells perfumey or harsh, then it will benefit from some airing. To air out the juice, just a) put the cap on b) shake it up, c) squeeze the air out a couple of times, d) put the cap back on. Do this once or twice a day until the objectionable smell goes away. If there is no objectionable smell, you shouldn't have to do this. (There are several MBV flavors that need this, and a lot that don't).
The second part is "steeping". Let it sit in a dark cool place. Shake it up whenever you can, or whenever you think about it. MANY MBV juices get much better with age.
As far as how long juices last, I recently dug out a very old bottle of FreedomSmokeUSA's Tiramisu dated 5/2/11 which tastes pretty good, and still has a strong nic kick. (Didn't taste anywhere near that good back in 2011, but back then it was in a cartrige-atty-cigalike setup, so there) Probably spent 2 years in the crisper drawer of my fridge, and the rest of the time rattling around in a box in normal climate. I don't think that juice is that fragile, although if you have exquisite taste buds you might disagree.
As always, YMMV!