Since it is for control of worms, I'm thinking that it is to be used continuously in the feed. Someone posted in one of the threads below to use it for at least 2 weeks. Here is the Durvet instructions form the manufacturer, along with 2 threads about it:
http://www.durvet.com/index.php?option=com_k2&view=item&id=380:strike-3
https://www.backyardchickens.com/t/929907/strike-iii
https://www.backyardchickens.com/t/...mer-why-wont-my-chickens-eat-it#post_12182658
http://www.durvet.com/index.php?option=com_k2&view=item&id=380:strike-3
https://www.backyardchickens.com/t/929907/strike-iii
https://www.backyardchickens.com/t/...mer-why-wont-my-chickens-eat-it#post_12182658