Depending on the settings of the platform on which you have posted your activity, you will once in a while receive an email with the request to extend your vacancy if it is still relevant.
The default setting is that you must renew after 60 days the first time. After that, you have to do it again every 30 days.
Upon receiving the email, you'll have 7 days left to do this.
If you choose to do nothing, the opportunity will automatically be marked as "filled". After this, you'll still be able to mark it is as "Open" in the opportunity overview (organisation page > opportunities tab)