Let's start by saying that free memberships never expire.
Basic and Pro memberships are valid for one year. 10 days before the expiry date, you will receive an email reminding you to renew your membership.
Renewal is not automatic. You'll have to do it yourself at Renew/Upgrade .
If you decide not to renew, you lose the advantages associated with your paid membership. On the expiry date, your membership reverts to a free membership. You lose access to your credits (if you have any left) but you will still be able to use the website and to download the free documents.
If, later, you decide to purchase a paid subscription again, the credits left in your account will be re-activated.