There's always room for discussion, but perhaps the section Transformation of the Roman Identity in Italy from our FAQ located in the sidebar will answer your inquiry.

Well, the rebel allies started minting coins with the legend "Italia" in the Social War of 91-89 BCE (ish - depending on what you want to mark as the end date). The die study for is Campana, La monetazione degli insorti italici durante la Guerra Sociale (91-87 a.C.). I consider this an aborted attempt at constructing an Italian identity - all of the hordes containing these coins contained many times more Roman coins; whoever was hording these didn't do it for ideological reasons. Anyway, if you're interested there are some 15 series of Italian coinage (depending on how you judge what constitutes a series; at least one series is comprised entirely of a single coin some people think is a fake, and other people think some series should be divided etc) and just short of 1000 known specimens, and use both Latin and Oscan. As far as ideology goes, “Italia” fits well with the establishment of a capital at Corfinium under the name Italica and the rationale given in the Rhet. ad Herr. as to Italian aims in the Social War – the destruction of Rome and the government of the empire as an Italian project instead of a Roman one (I can't put my hands on it at the moment, but the Rhet. ad Her. has a passage about the allies ripping empire from Roman hands – I'll try to chase it down later if the quote is wanted).

Italia shows up again in the Res Gestae in which Augustus claims to have gained the support of tota Italia probably before the Battle of Actium in 31 BCE - exactly what this means is a matter of debate. Syme thought this excluded communities loyal to Antony - which is troubling if right. It doesn't seem to me that "tota Italia except the bits loyal to Antony" is a useful boast except for rewriting history late in Augustus' life, which I suspect he was totally capable of.

Augustus was also at some point responsible for administrative changes to Italy that distinguished Italy from the Provinces, especially in division of the peninsula into administrative districts with regular administrative offices across the peninsula and the inclusion of Cisalpine Gaul to the Alps in Italy proper (Williams, Beyond the Rubicon is a good survey of this). Williams wants to make the inclusion of Cisalpine Gaul at 42 BCE, and the organization of the peninsula into administrative districts about the same time, but I don't think we have any evidence to support that directly. The argument as I recall it has to do with the ratification of Caesar's acts, appointments, plans etc and the organization of Italy is supposed to have been included in that. I admit that that is as good a position as any other, but it's not strong for establishing a date of when people started being Italian.

The terminus post quem might be Claudius' speech to the Senate about opening magistracies to the Gauls delivered in 48 CE. Apparently Claudius' words are preserved verbatim in the Lyon Tablet, and Claudius here twice mentions the Senate as an Italian, not Roman institution. “What? Therefore an Italian senator is not preferable to a provincial one?” is an example of this. I don't think it can mean a Roman senator is preferable to an Italian one, but an Italian is better than a Gaul if we are forced to be inclusive, as Italian elites had been eligible for magistracies for centuries. There's a later reference to the border of Narbonnese Gaul; the impression I get is one that is drawing geographical limitations on the background of the Senate based on Augustus' organization of Italy. The Lyon Tablet hasn't got very much scholarly attention aside from its importance for Tacitus' account of this same speech though. Unless I've overlooked something?

Or you could be talking about the 19th century unification. That's no fun though.