Total milk per day is one quart or 7 gallons a month. If your doe is pregnant, then her milk will continue to dry up and at 10 months you should stop milking. If your doe is not pregnant, she may continue to produce milk for up to 2 years. It really just depends on the individual goat.
A Simple Guide to Raising & Milking Goats – Weed ’em & Reap