I believe that the first possible vote expiration is a year from the hard fork. This line in the code seems to confirm that belief as it simplifies to mean: HARDFORK_1_25_FIRST_GOVERNANCE_VOTE_EXPIRE_TIMESTAMP = HIVE_HARDFORK_1_25_TIME + HIVE_GOVERNANCE_VOTE_EXPIRATION_PERIOD
where HIVE_GOVERNANCE_VOTE_EXPIRATION_PERIOD = 1 year.