Schengen visas (which are what Italy issues) can be issued with a validity of up to 5 years.
It is up to the issuing consulate how long a validity they will issue visas with. Visas with long validity are only supposed to be issued to travelers with a known travel history and a demontrated need to make frequent and regular visits to the Schengen Area. Different consulates interpret these guidelines differently.
The vast majority of first-time applicants will only get a visa that covers the particular period of the visit they document in their application, perhaps extended with a week or two at each end to allow for minor changes of plans.