By default, job posts displayed on your job board will expire after 60 days. If you wish to change the duration that a job post is visible, navigate to Settings > Apps > Jobs and customize the "All job posts must have the same default posting duration.
Note: Job posters will still have the ability to customize the closing date of jobs they submit. If the default expiry date comes before the closing date, the job will be removed at this time. If the closing date (set by the job poster) comes before the default expiry date, the job will expire at this time. The sooner of the two values will determine when the job is removed from the board.