Peaceful & democratic: freely resigning, handing over the presidency to someone else, conceding defeat in an election.

Non-peaceful / non-democratic: death, imprisonment, coerced into resigning, defeat in an election/decision he contests, Ukraine ceasing to exist.

Whether an election result is "contested" will depend on Zelenskyy's official response. If his supporters contest a result but he accepts it, this will count as uncontested.

Re-election would not count as the end of his tenure.

The market will close when Zelenskyy ceases to be president of Ukraine.