A single molecule of mRNA can code for multiple numbers of proteins.  The actual number coded for by each mRNA transcript is dependent upon the length of time a transcript exists within the cell before it degrades (this phenomenon is called "turning over").  The stability of the transcript is measured as mRNA half-life, and ranges from as short as a few seconds to as long as several days.

Most eukaryotic mRNAs terminate in a sequence of approximately 200 adenine residues, called the poly(A) tail.  Over time, the poly(A) tail degrades, and this contributes to mRNA half-life, since mRNA molecules degrade more rapidly as the poly(A) tail shortens.

The 3' noncoding region of  mRNA may also play a role in half-life.  3' regions rich in adenine (A) and uracil (U) residues appear in mRNAs transcribed from genes that are transiently (only occasionally) expressed, and appear to act as signals for selective degradation.  AU-rich 3' regions are recognized by 3' exonuclease, which degrades mRNA from the 3' end.

It has been shown that the half-life of mRNA can play a role in oncogenesis.  The normal cellular c-fos gene codes for mRNA with a 67 bp AU-rich 3' sequence, conferring the mRNA a short half-life.  The retroviral v-fos oncogene codes for mRNA lacking this AU-rich sequence.  It was also shown that if the 3' AU sequence is removed from the c-fos gene, it is transformed into an oncogene.