The subscriptions to Open Archives Plus, Genealogy Online Plus, Family Archivist Plus and Stamboom Forum Plus are all annual subscriptions.
Some time before the end of the year you will receive an email announcing that your subscription will be automatically renewed. So you can continue to use all the functions that the Plus subscription / the Plus subscriptions offer you without any worries.
If you do not want to renew your subscription(s), you can revoke your direct debit authorization via the link mentioned in the e-mail. The Plus subscription will not be renewed after this and will be converted to the free Trial subscription, which lacks the extra features of the Plus subscription.
Some time before the end of the year you will receive an email informing you that the subscription will not be automatically renewed and that you must take action if you want an extension. The email message will contain a link to renew your subscription(s). This will take you to the Subscription page where you can indicate which Plus products you want and pay for them again via iDEAL, PayPal, credit card, etc.
The start date of this new subscription starts one day after the current subscription expires (or today if you did not previously have a subscription or the previous subscription has already expired).