how many compensation days does apple give when i haven’t paid my icloud storage subscription?

There is no grace period. You don't pay your subscription, your services are cut off and your account goes to the 5gb free storage limit. If you are over that, you won't be able to backup, or get new mail from your @iCloud.com account. You have up to 90 days to resolve the overage of storage before Apple reserves the right to start deleting content.

Downgrade is usually immediate. The 30 day window you speak of is for the temporary iCloud storage if you are jumping between devices after having purchased a brand new unit. The files do remain intact up to 90 days after the overage, you just can't add anything to it. If you start the subscription again in that time frame, everything returns to normal as if it never happened.