iCloud storage

So, how long are we supposed to pay for it?

For as long as you want to maintain access to the added storage.

You renting the storage space, you do not own it. Just like you pay the rent as long as you wish to live in a particular apartment. You pay for the storage for as long as you want to have access to it.

This is a monthly fee. You can cancel your subscription anytime and downgrade again.

Check this article for more details on that:

Buy more iCloud storage - Apple Support