I had a brief chance to jump on the test realm last night and check out half of a PvP battle. My level 10 Witchdoctor vs a level 16 Swashbuckler. From that brief experience I can say . . . that PvP works about how you'd expect it to.
You go into one of the PvP-designated buildings, stand on a sigil and, when there is more than one person, the battle counts down and begins. The victory conditions are "Defeat all enemies", so my initial thought was to focus fire the pirate, as I know that's what would hamstring me the most, especially at that low a level. I was at a slight disadvantage though, since I hadn't done two of the promotion quests I really should have, so my crew was a bit underpowered.
When the battle starts, it's randomly decided who goes first (in this case, the Swashie) and while was thinking about his moves, I could see the count-down timer, so I'd know how long before he'd be forced to stop. It appears to be the same length as the regular battle timer. Then it was my turn. We went back and forth, it appeared that he would win, though I made a dent, and then my internet went down.
Hopefully, I'll have another chance to try it this evening. As I understand it, there are currently no rewards or penalties for PvP at the moment, so it's just a diversion for now. All the other trappings they will add later after they have a chance to see how balanced things are. For now, it feels like it should and I look forward to testing it again. It is werid playing against another person and it will be interesting to see what sort of strategies emerge.