Your billing cycle starts and ends on the same day that you started your subscription.
To see your billing cycle:
On the top right hand of the screen, next to your account name, you will see a small drop down button. Go to My Account. On the "Account Info" tab and look on the right hand side to see when you were billed and how many surveys you've used. This will help you know if you want to upgrade to a higher subscription plan, or just stretch out the number of surveys you use.