Listings are valid for 180 days. Once a listing expires, you will have the option to relist at an 80% discounted token price. For your convenience and to ensure you get the discount, your listings will be set to auto-renew, but on February 9 you can change that default by going to Account Settings under the header Selling and unticking the box  - 

That default will be used for when you list new items for sale (and where you can override your default)  -

You can also set or unset auto-renew for each item in your Seller Dashboard. There you can also filter to see which items are set to auto-renew within your seller dashboard.