I've read a lot of places that say that Darjeeling tea, especially first flush, does not keep as well as other black teas, usually citing a vague notion that it is "delicate".
My personal experience of this, though, has not been the case...I find Darjeeling tea, even first flush, tends to keep a year or sometimes even more, if stored properly. Also, if a tin or bag is sealed, I find storing it unopened doesn't really result in much degradation of flavor...but if you're constantly opening it up and closing it, it loses flavor more quickly...especially if it's a small amount of tea in a large container.
I also find it varies hugely by individual tea.
Airtight and dark is important for storage (unless you're talking aged teas, they need to breathe). But the amount of time that tea stays fresh is a matter of controversy...and it's probably a bit of luck too, the individual batch of tea you have, the temperature and humidity in your home, etc.