If I buy 50gb and pay 0,99€,this payment will last as soon as the 50gb last or I have to pay every month and for how long?

You would actually be renting iCloud storage space. The payment would continue monthly for as long as you wish to use the service.

Do note that cloud storage does not add storage space to an iDevice.

You will have to pay that every month and for as long as you want to keep using the 50GB iCloud storage

iCloud storage is purchased on a month to month basis. You pay for it as long as you need it. If at some time you don’t need that much space, you can downgrade to the free 5 GB plan.