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