When the free trial period is over, your account will automatically be charged with the monthly subscription rate if you do not cancel by the last day of your free trial period.  A “day” is measured from the time you start your free trial period and ends 24 hours later.  For example, if you start your free trial at 3 p.m. on a Monday in your time zone, and your free trial is two days, you have until 3 p.m. on Wednesday in your time zone to cancel before your paying subscription will begin. 

For information on subscription plans, benefits and rates, check out this article
Please note: The free trial offer is only for new, first-time CBS All Access subscribers.