Hilchos Issurei Mizbeiach 2:13

It can be determined whether water in the animal’s eyes is a temporary or permanent blemish as follows: if the animal ate fresh grass from Rosh Chodesh Adar through the first half of Nisan and dried grass from Elul through the first half of Tishrei but the condition did not resolve, we see that it is permanent.

Hilchos Issurei Mizbeiach 2:14

The minimum volume of fresh and dried grass that must be eaten in the appropriate seasons is a fig’s volume before its first meal every day for these three months. It must be eaten daily after drinking and the animal must be permitted to wander the field while eating. It should not be alone; rather, there should be another animal with it. If after this treatment the condition is not resolved, the blemish is surely permanent. If part of the treatment was omitted, there remains a doubt and the animal may not be offered or redeemed.