[ken3009]

what do you reckon is the shelf life of a 2ppm nitrate solution in water? of course its kept in a suitable plastic container.

i know bacteria tend to chew up the nitrates.

3 months maybe?

[Arkcon]

It really depends on the environment in your lab, and what your application is.  You won't find a catch-all rule for how long it will last.  Very dilute solutions are very prone to microbial attack, especially if it contains a nutrient.  You may have to make it fresh every day, and compare it to a batch of your stored solution, and determine for yourself when a batch has gotten too old to use.  Then, store that data, in order to justify your chosen shelf life, to anyone who asks.