Only you can decide!
The Pompei site is some 164 acres in all, and the authorities there say 30% of it is visitable....
These people have clearly walked most of it:
http://pompeiiinpictures.com/pompeiiinpictures/index.htm
The official website is only available in Italian at present:
http://www.pompeiisites.org/index.jsp?idProgetto=1
... but they've timed itineraries for Pompei - of 2 hrs, 1/2 day and the whole day...
http://www.pompeiisites.org/Sezione.jsp?titolo=Quanto+tempo+hai+a+disposizione%3F&idSezione=58
... conveniently with versions arranged for each of the 3 entrances... like this, the one for 2hrs:
http://www.pompeiisites.org/Sezione.jsp?titolo=Due+ore&idSezione=69
... and also some good maps, links at the end of the page here:
http://www.pompeiisites.org/Sezione.jsp?titolo=Info+visita&idSezione=1127
Perhaps also worth having these, of the town and its public transport, and of the site:
http://www.unicocampania.it/index.php?lang=en¢er=inside&colonna=turisti&action=mappe_pompei
If buying tickets, the €20 2 day version might be a good idea - which will also get you into Boscoreale, but probably covers only 1 entrance to each site....
http://www.tripadvisor.co.uk/ShowTopic-g187785-i421-k3723054-Not_only_Pompeii_Herculaneum_Oplontis_and_Boscoreale-Naples_Province_of_Naples_Campania.html
(or even the €27 3 day Artecard, 2 prepaid admissions, then reductions thereafter with use of local transport included, if you've not seen that?)
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Particularly if here over a weekend, there's a link to some great advice from another TA member who visited recently, start on:
http://www.tripadvisor.co.uk/ShowTopic-g187786-i1888-k5395479-Making_the_most_of_your_visit_to_Pompeii_as_in_How_To-Pompeii_Province_of_Naples_Campania.html
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For Herculaneum, note the links to lists of buildings etc that are open (visitabili) and closed (non-visitabili), at the foot of this page
http://www.pompeiisites.org/Sezione.jsp?titolo=Organizza+la+tua+visita&idSezione=93
Peter