The half life of specific mRNA is studied in concern to study the mRNA constancy by monitoring changes in steady-state level of mRNA in the cytoplasm. Each individual mRNA has inherent stability under a given condition and it may change in response to variety of external stimuli. The transition of mRNA constancy renders a potent way for regulating gene expression during various physiological changes. Follow the link given below to an article related to the determination of half-life of specific mRNA in mammalian cells. Hope it would help you. All the best.
Article Chapter 17 Messenger RNA Half-Life Measurements in Mammalian Cells
In principle, mRNA half-life is measured in cell culture by first inhibiting the transcription by RNA Polymerase II, generally with a chemical inhibitor such as alpha-amanitin, and then measuring the decay of a specific transcript over time with a quantitative assay such as RT-PCR in real time.
Thank you Mr. Gregory Dressler for the explanation of the principle, but i am always confused because after the inhibition of RNA Pol II it is possible that cell increase the stability of mRNA, because RNA Pol II transcribe miRNA that are responsible for mRNA decay.