It can vary depending on the age and breed of the goat. Well bred and kept goats can hold up to a gallon of milk, and can be milked twice a day. Lesser breeds may only be able to hold a half or quarter gallon at a time.
This largely depends on the individual animals, but it is contained in her udder, so you can look there; she will not produce over about 1/4 a gallon per day.
