How long you have to make a payment for a storage plan on iCloud?

Apple does not mention a grace period.

https://www.apple.com/legal/internet-services/icloud/en/terms.html

III A. "The applicable storage fee for an upgraded storage plan will take effect immediately; downgrades to your storage plan will take effect on the next annual or monthly billing date. YOU ARE RESPONSIBLE FOR THE TIMELY PAYMENT OF ALL FEES AND FOR PROVIDING APPLE WITH VALID CREDIT CARD OR PAYMENT ACCOUNT DETAILS FOR PAYMENT OF ALL FEES. 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. If you want to designate a different credit card or payment account or if there is a change in your credit card or payment account status, you must change your information online in the Account Information section of iCloud; this may temporarily disrupt your access to the Services while Apple verifies your new payment information. We may contact you via email regarding your account, for reasons including, without limitation, reaching or exceeding your storage limit."

and:

Downgrade or cancel your iCloud storage plan - Apple Support

March 2021 post by ASC contributor Michael Black

https://discussions.apple.com/thread/252568292?answerId=254839872022#254839872022

- "When people cancel a subscription, they get an email saying they have 30 days to transfer their data for safe keeping, after which data will be purged to fit within 5GB."