When you purchase word credits you have two options-
A yearly subscription:
allows you to roll over up to 20% of the plan's unused words into the next year's contract. The contract term is 12 months. Minimum of 75 words per order. If the subscription is canceled the plan is then locked and credits can not be used.
Words expire 12 months from the purchase date. There are no roll-over credits, once the credits expire after 1 year.