The amount of each RNA molecule in a given cell at a given moment results from the equilibrium between the rate of synthesis and the rate of degradation.

The first is composed of the rates of transcription and processing.

The second depends on both intrinsic and regulatory factors. 

In general, for mRNAs, there is a balance among translation and degradation, even tough cases of "inactive storage" of mRNAs exist (as for example for mRNAs encoding synaptic proteins in neurites).