If you subscribe to one of the monthly plans, then you are paying month to month and you can cancel at any time...say 6 months. Obviously, we hope you won't do that, but you can cancel. The downside is that the monthly rates are more expensive on a per month basis than are the prepaid plans.
If you subscribe to one of the prepaid plans, you can save considerable money and lock in a lower rate for one, two, or even three years, but in order to cancel a prepaid plan, you must do it in the first 30 days after it takes effect.
All of this is spelled out in our payment terms which you can read about here at this link.
I hope this is helpful for you.