How long can I work in Italy as an expat?
What nationality are you, and what kind of work do you plan to do? These two factors will affect the work permit requirements quite drastically.
If you are from an EU-member state, you don't need a work permit to work in Italy. However, if you're from a non-EU country, your work permit will be valid for anywhere from six months to two years, depending on what kind of work you're doing. Also please note that work permits in Italy are tied to one employer, meaning that if you change jobs, you will need to apply for a new work permit.
For a detailed breakdown of the process, see Work Permits in Italy.