Purified RNA shoud be stored at –20C or –80C in RNase-free water. When isolated using QIAGEN systems, no degradation of RNA is detectable for at least 1 year under these conditions.

For ever, or at least your life-time, if it is clean and stored properly.

It is quite stable if stored in dilute TE (1 mM Tris/0.01 M EDTA) at a concentration of 1 mg/ml.  For best stability, store as an ethanol precipitate. I have used RNA stored this way at least 5 years after initial extraction with good results.

thanks all. so, RNA much more stable unlike miRNAs. right? also, any way to test the stored RNA before doing qPCR?

RT for long-term storage is the best option, but you still need to store the cDNA in higher concentration of TE buffer (10 mM Tris/1.0 mM EDTA) as low pH can destroy even cDNA

Testing stored RNA for its quality is a very subjective thing and depends on the context.

You could run RNA gels under denatured conditions and look at the pattern - generally speaking, if you are getting two strong bands large and small ribosomal RNA, one assumes that the quality is decent enough.

If the RNA was already purified to separate/enrich for mRNA with poly-A tails, all you will see is a smear. You will have to compare the appearance of the gel run at the time of purification with what you see now.

Relatively speaking, qPCR is the quickest and a reasonable way to assure that you have useable quality RNA. Remember to amplify a few housekeeping RNA molecules, ideally representing a range of relative abundance.

By the way, RNA is RNA, there is nothing magical about miRNA except the quality of preparation and storage conditions.