Our subscriptions (both Basic and Pro plan) will renew automatically after 1 year. If you would like to have it only for 1 year, you can cancel the auto-renewal in your account settings: https://www.mentimeter.com/app/subscription.
The cancellation only cancels the auto-renewal - you can of course still use Mentimeter until the end of the period you have paid for! Therefore, you can make the cancellation whenever you like.
Once the subscription have been cancelled it's unfortunately not possible to reactivate that subscription, but should you chose to continue on with Mentimeter after a year you can easily purchase a new subscription.