Retargeting campaigns are designed to run continuously. Think of it as a tool that constantly makes efforts to help you bring back all those potential customers that leave your website every day. 

If you could know who they are, what they like, and have a way to not lose touch with them, you would certainly do it.

This is why your retargeting campaign is a living integration to your website that helps you keep in touch with your customers even after they leave your website.