Your billing period starts on the day you activate your service. All base plans operate on a 30 day billing cycle. For example, your base plan purchased on January 1 will expire on January 31st. All add ons carry a separate 30 day validity from date of purchase. So, any add ons purchased on January 15 will extend through the base plan billing cycle and expire on February 14.
Sign in to your account to see when your billing cycle ends.