Is iCloud storage billed monthly or annually?

I noticed a $40 charge on my bank account. I have never noticed it before. I checked into it in iTunes and it stated th charge was for iCloud storage 20B. Is this an annual charge or monthly? You would think I would have noticed it if it was monthly, but maybe I just haven't been looking as closely at my bank statements as I should. Finally, do I really need to be paying for this service? Thank you!