It can take up to 1 year(for long standing infections) - doing parasite cleanses every 90 days. Pork tapeworms are the main culprits of eye parasite problems. They live in the head area and can affect the eyes.
Usually by the time the eyes are infected - the parasite infection has been around for quite some time. As with any other "drug" - the natural chamucals found in herbs can only kill a certain amount of parasites at a time.
This is why it might take 3 cleanses along with detoxing for up to a year.