iCloud Storage is automatically charged to the Payment method on file for your account. So there's no possiblility of you forgetting to pay for it.
If however, your payment method cannot be charged and there is not enough iTunes Credit in your account to cover the cost of the storage then:
If Apple is unable to successfully charge your credit card or payment account for fees due, Apple reserves the right to revoke or restrict access to your stored Content, delete your stored Content, or terminate your Account.
Read more here:
Legal - iCloud - Apple
There is a time buffer between not being able to charge and deletion or removal of files, however, the time it takes is not known so don't rely on it.
You should make sure your payment method can be charged on the date of the renewal, or that there is enough iTunes credit to cover it.