The Recurring Payments add-on allows you to add a recurring option to your products. You can set these to automatically renew or require the customer to purchase the product after the time has expired.
To enable Recurring Payments, you'll go into your Advanced settings of your product creation page and find Recurring Options. Check the Enable Recurring Payments box.
Once you have enabled the recurring options, you'll be able to set how often the product would recur.
If you want the payment to be charged automatically when it's time to renew, check the Enable Auto-Renewing option.
With Auto-Renewing enabled, you can also offer a free trial of the product. To do so, check the Enable a Trial Period setting and then select how long you'd like for the trial period to be.
If the product is Auto-Renewing, the customer can cancel their subscription by going to Account > Purchases and finding that purchase. They will see a link to cancel there. Depending on the payment gateway used to make the purchase, the user may be taken to the payment gateway to cancel the subscription.