The answer really depends on how much time you want to spend exploring the site. Pompeii is huge, and there is a lot to see. If you just want to walk through the ruins and get a general sense of the place, you can probably do it in about two hours. But if you want to really take your time and learn about the history of Pompeii, you could easily spend four hours or more at the site.
Plan your visit to Pompeii

The easiest way to get from Naples to Pompeii is by the Circumvesuviana train. The Circumvesuviana is a regional rail line that runs from Naples to Sorrento, with stops in Pompeii and Herculaneum along the way. The stop for Pompeii is called "Pompei Villa dei Misteri" and the entrance to the archaeological site is just steps from the train station. Tickets can be purchased at any of the Circumvesuviana stations and the journey takes about 40 minutes.
Circumvesuviana train schedule from Naples to Pompeii

You can purchase tickets for Pompeii and Herculaneum at their respective ticket offices at the site entrances or online. The most convenient option is to purchase combination tickets online for both archaeological sites like Pompeii + Vesuvius or opt for a Pompeii guided tour.