Depends on the flavoring used. In my experience, strong, dark, or earthy flavors like coffees, chocolates, and tobaccos generally benefit from steeping indefinitely (or at least as long as you're able to not vape them up

). Lighter flavors like creams, bakeries, and some fruits also benefit, but may suffer from the "flavor fade" phenomenon if left steeping for extended periods of time in uncapped or breathable containers like LDPE bottles. I don't steep anything for longer than 4 weeks max. If there is no alcohol content in your juice, there is no need to remove the cap. Failing disclosure by the manufacturer, a sensitive nose can detect the presence of ethyl alcohol in e-liquid: It imparts a sharp, sort of tangy aroma to the flavoring. Difficult to describe.
Without fully understanding the chemistry involved, I think that juices with powerful flavorings in them get better over time simply because those flavorings tend to mellow out or fade into the base mixture. Super-concentrated flavorings (generally ones that use alcohol) will actually start out on the weak side when fresh, intensify as the alcohol evaporates, and then begin to gradually decline from there. At some point along the arc of that gradual decline is where a particular juice might be at its peak flavor performance.
It's a fascinating subject, and I wish we had more scientific explanations on exactly how and why the flavors in our juice change. My observations are far from scientific, but are based on nearly 7 months of mixing my own juice with various brands of flavorings at various concentrations, and storing/aging/steeping in LDPE, PET, and glass bottles. As always, everyone else's MMV!