I don't have a reference but I believe that N (in solution) is stable for several months or even years but refrigeration should help prolong the shelf-life.
I store mine frozen - which is usually not recommended because 1) the glass vial could break (it did not) and 2) precipitates may occur (which is something you do not want with a drug you will inject, but in this case I don't care as I will swallow it).
It is also in my freezer because I am hiding the bottles under a thick layer of delicious, home-made, green tea-flavored, ice cream. It will be an ultimate test of my resolve on my last day as ice cream is one of the most potent OTC antidepressants known to man and I will not let it to waste!