It is entirely up to the players to set a limit on how long the campaign lasts. A campaign could go on indefinitely, but it is often better to end it once a player has established a dominant position. In this way a winner can be declared and a new campaign
started.

Saying that the winner is the first to conquer three territories, or get so strong that the other players refuse to fight him, is a good method for your first campaign. By then you'll have developed your own ideas about how to 'improve' this campaign system, and will probably be thinking about adding in new rules for things such as spies, assassins, ambassadors, etc. The possibilities really are endless.