I personally thought that the date of 510/509 BC with the establishment of the Consulship a bit too pat, since most, if not all the other offices and laws were not set yet, for example the 12 Tablets were a few years later.
Well, this whole period of Rome's history is shrouded in myth. Â I mean, can we really believe that everything that Livy records in his first five or ten books went down exactly the way he says it did? Â Sometimes I'm not sure even he believes it. In any case, the establishment of the Republic does seem to have taken place around 500 BC.
As for the demise of the Republic... I would put its start during the period of the Gracchi, since they exposed the fundamental problems of the Republican government vis a vis the governing of an empire... Â And how "independent" was the Senate anyway during the reign of Marius? Sulla? Pompey? the Triumverate? Â The senatores were just hollow men rubberstamping the decisions of the current 'first man'.
Yes, certainly, that's when it all started. Â Yet A) The Gracchi failed, and were assasinated; and B) the Romans were still freely electing consuls and tribunes until Caesar took over, and the fortunes of each party waxed and waned; in other words, politics was alive, if not well. Â The Republic lived on after the Gracchi. And, in any case, the Gracchi's heyday was about 120 BC. Â So even if one dates the demise of the Republic to the Gracchi, that still gives it 390 years of life before it became a tyranny.