Most protocols suggest that tissues should not fixed with formalin past 48 hours to prevent over-crosslinking proteins, which may be irreversible and thus it may be difficult to extract RNA.

Thavarajah et al. (J Oral Maxillofac Pathol. 2012 Sep-Dec; 16(3): 400–405) wrote:

"The initial cross linking is completed by 24 h to 48 h after penetration while the latter may take about 30 days to generate the stable covalent cross linkages. The initial phase of the reaction is reversible while at the latter stages the reaction becomes irreversible when there is high number of covalent bonds formed."

Go through this article will help you 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3077733/

There are two issues - getting the RNA out from the protein matrix, and keeping the RNA intact.  Assuming the formalin is well-buffered, the RNA will stay intact for quite a while, but if the solution becomes acidic, as formic acid is created, RNA will depurinate and fragment.  In terms of removing cross links - the sooner the better, but you can get RNA for a very long time.  RNA integrity is reduced in subsequent steps of fixation (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3077733/), but when sequencing 1918 influenza virus, Taubenberger et all were able to get highly degraded RNA (but still could be sequenced), with difficulty) from 80 year old paraffin blocks.