What happens when the 30 days expire?
Short answer: If the timer runs out before Brencis is defeated, the Time Runs Out failure ending triggers: Coen's family is sacrificed and the rescue is lost.
Consequences
The campaign gives Coen 30 in-game days to defeat Ambrus, Bakir, and Xanthe, draw out Brencis, and rescue the family. Letting the deadline pass before the final victory changes the castle assault and epilogue to the failure route. It does not award an ending trophy.
You can still assault Griefberg Castle after the deadline, but the family cannot be saved. Keep a separate save before the final quest if you want to compare this ending with the four standard endings and the two early endings.
Planning note: Time advances through quests and certain dialogue/actions rather than free exploration alone. Check the hourglass icons in the journal and reserve the final days for the vassal and Brencis sequence.
Sources: PowerPyx all quests, PowerPyx seven endings, and PC Gamer endings guide.