Just double checked at a dairy site. They claim the "average cow produces 53 pounds of milk a day." .Ok, divide that by 8.5 ( 8.5 pounds to one gallon) and you get 6.2 gallons a day. Personally, I think that's a little low ... but they must average in Jersey's, Guernseys, and pasture feed. A lot depend on breed and feed! Also, the two milkings are not equal. 3/5 or better of the milk produced will happen in the early morning, cause the cow was resting most of that time. During the day they are walking around, chewing their cud, eating, drinking, being nosey, destroying stuff, licking stuff, blah blah blah. Burning energy.
Remember too, that a cow will produce like gangbusters after the first week or so, building up in the first ninety days, then the rest of the lactation is down hill! Often with the last month or so getting milked more to ensure her health than for milk production. Then the cow needs to rest before her next calf, and will be out of action, so to speak, for 45-65 days.
Also, if your cow grazes for her primary food source you milk will taste like grass.
FYI, before you think farmers have it made, they are getting $13 per 100 pounds of milk in my area right now. That's $13 for 11 gallons of milk. It currently costs over $20 to produce that milk.Mostly because of fuel prices. Two farms here have just gone belly up.
"There ain't no vaccine for stupidity* Maxine