Upvote:2
The official page of the Consulate General of Italy in Philadelphia (click English in the top right corner after you open the page if the page is in Italian) has all the informations you need (forms and instructions are in the bottom part of the page) prepare all the required documents and schedule the appointment.
From their FAQs:
How long will it take to get my visa?
It may take between 3 to 21 days depending on the applicant's nationality.
And regarding the right time to apply for a visa
Since during the months of: May, June, July, August, November and December the number of visa applications increase, it is therefore advisable to apply at least 3 weeks in advance. Note, however, that applications cannot be processed earlier than three months before your planned departure date.