There's a set number of rounds. I've played with 10 rounds, I've played with 20 rounds. That would be determined once we settled on a subject and had a general idea of how many people were playing.
Once the final round ends, we open voting to players and observers and hold a single-elimination tournament where each "island" goes up against another "island", the winners then play each other and so on until there is one champion.
As I mentioned to Skulker, "winning" is generally secondary to picking things you like, but there's no shame in trying to pick a collection that would get the most votes.