if i upgrade my storage, how long would i have to pay it for?

You pay for iCloud storage every month until you cancel/downgrade the additional storage. Think of it like renting a storage locker. You get a fixed amount of storage space, that never changes, for a fixed, monthly fee.

You will pay for it every month for as long as you use the additional storage. If you cancel the upgraded storage, then you quit paying for it. When you use iCloud for storage, you are renting space on Apple’s servers to store your data and backups. If you decide that you no longer want to rent the space, you will quit paying for it but you will lose the additional storage space that you were renting and you will keep the free 5GB of storage that Apple allows for each Apple ID/iCloud account.

I always use the storage locker analogy because that how iCloud storage works. If you are renting a storage locker, you get a fixed amount of storage space for a monthly fee. If you decide that you no longer need the locker, you quit paying the monthly fee, but you also lose the storage space.

Life long until you decided to cancel iCloud plan currently I am using base one

Well, let’s get to the point. When you say ‘upgrade my storage’ do you mean the internal storage of your iOS device? That cannot be increased by subscribing to more iCloud storage. iCloud does not give you more room on your device.