There is a system-wide setting that can be used to control how many days jobs are published for on the website. Go to the admin menu (top right) and select Jobs. Here you will be able to enter the number of days jobs will be published for.
This is a default value so in the example below if I post a job it will be live for 30 days before becoming unpublished. You can override this setting by manually un-publishing a job, or if a job expires and you want it to remain online it is advisable to Renew the job. You can see a quick guide on how to renew a job here.