Xenophon, Polybios (with an old handle), and I discuss the use of "empire" equivalents in this earlier post. But as for historiography, that division appeared rather quickly. Recall that Julius Caesar rose to power in a civil war against Pompey and the senate, and that makes a good, non-arbitrary historical division. Our most complete sources for the early empire, such as Suetonius and Tacitus, begin with Julius or Augustus Caesar. Others like Livy divide books of their work at this point in history.