ICloud Expiration

Should your iCloud subscription expire (payment didn't go through, or you cancel) your storage will go down to the free tier. If you have excess data beyond that, after 30 days you will lose that data.

So basically, you get a 30-day grace period before you lose your data.

Hi, Ryan,

Thanks for visiting the Apple Support Communities. I understand that you would like to know what would happen to your content if you missed paying your iCloud Storage for a month and were reverted back to the free 5GB storage. I'm happy to help!

Per the information below from Downgrade your iCloud storage - Apple Support, your existing data would be fine. However, no new data would update or sync until you managed your content to fit into the 5GBs of data:

"If you downgrade or cancel your plan and your stored iCloud content exceeds your available storage, new photos and videos don’t upload to iCloud Photos, files don't upload in iCloud Drive, Messages in iCloud don't update, and your iOS devices don't back up. Learn how to manage your storage by deleting the information that you don't need anymore."

While you can sync and store your data in iCloud, it is not intended to be a back up service. If you'd like to ensure your pictures and other important data is safe, you should make an archived copy on a computer or external hard drive: Archive or make copies of the information you store in iCloud - Apple Support.