When a thumbnail expires it will be removed from our system. But if the URL is requested again a new thumbnail will be generated and the expiration date will be reseted.
Today you create a thumbnail by sending this request using the free plan: http://api.rethumb.com/v1/width/100/http://images.rethumb.com/image_coimbra_600x600.jpg
After 6 months that thumbnail will be deleted. But if you access it again a new thumbnail will be generated (just like the first one).