"What are each Faction's requirements to win the campaign? Can only 1 Faction win? What do the players of that Faction get for winning the campaign? How long does each campaign last, on average?"
Nobody “wins” a campaign – you get a set of campaign objectives specific to your faction (and almost always directly opposed to another) and you try to complete as many as possible during the campaign period. At the end there is a summary of accomplishments tied to rewards for those that participated, and some campaign objectives have an affect on the server or give bonuses for the next campaign.
The Objectives themselves are of varying lengths and pop in and out during the campaign time.