The Blightstorm serves as the meta-progression timer for the entire World Map Cycle, but within an individual settlement, you are racing against the Queen's Impatience. As the years pass, Hostility intensifies and the Storm seasons become harsher, making villager Resolve harder to maintain. If the Queen's Impatience bar fills completely, the settlement is lost to the Blightstorm.
Victory is achieved instantaneously the moment you reach the required Reputation threshold, which varies depending on the difficulty. Filling the Reputation bar immediately secures the settlement, and you'll collect your rewards, including valuable Citadel Resources used to upgrade the Smoldering City. Every successful expedition pushes back the Blightstorm's reach on the World Map, granting you more time to explore before the Cycle resets.