An open seat is to be filled in accordance with state law. Ohio's election law requires the Governor to issue a writ (or order) of election to direct a special election "when a vacancy...occurs." (See Ohio Rev. Code 3521.03). But the law does not require this be performed within a specified time frame.
This means observers will have to wait until Boehner officially vacates his seat for the process to begin, and for the Governor -- who is current presidential candidate John Kasich --to exercise his discretion.