How long to pay

You pay monthly for as long as you want to continue to use the storage. It is similar to renting space.

If you want to see if you can reduce the storage you are using, see: Manage your iCloud storage - Apple Support

If you decide to cancel, you do that by downgrading to the free 5GB plan. See: Downgrade your iCloud storage - Apple Support

It is a subscription for additional storage. Like any subscription, you pay continually (at whatever frequency, in this case monthly) until you decide you no longer want the subscription and cancel it.

iCloud storage is like renting an apartment...you pay for it every month you are using it.