Let me give a commo-sense answer, which is that the later an event is recorded after it occurs, the less reliable that account will be.  However, there were ancient societies which were actually quite good at maintaining oral histories over multiple generations.  This has been confirmed by much careful study by scholars.  So, the quality of histories recorded after the fact will vary widely.  In order to address the reliability of oral histories which are turned into written accounts 1-3 centuries later, we need to look at the nature of the evidence related to that history.  Were they contained in oral histories?  Did those who recorded the events have some motivation to create a false or highly exaggerated account?  So, in one sense, this is an easy question, but in another sense it is a complex issue that should be studied carefully.  Each example should be taken on its own merits.

John Oakes