An Italy Tourist Visa is valid for the same amount of time that all Schengen visas are: a maximum of 90 days within a six-month period.
In some cases, your visa can be issued for less than 90 days, depending on the type of Italy Schengen Visa you receive (single-, double-, or multiple-entry).
You cannot stay in Italy (or any other Schengen country) with a Schengen visa for more than 90 days. This is why it is called a short-stay visa.
When you receive your visa, the visa sticker on your passport informs you on how many days you can spend in the Schengen area. It also informs you when you can enter the Schengen area and what is the latest date you can leave.