First of all, it takes approximately 8 pounds of milk to make a gallon of whole milk. (we'll leave it simple for now, and not worry about trying to convert it to skim)
Our milk gets picked up every day, and the milk truck driver writes the weight based on the measurement of the liquid in the tank. Our cows will vary what they milk from day to day, depending on the weather and a variety of other factors, but overall stay pretty consistent from day to day until we get major weather shifts (like summer heat).
For instance, today our cows milked 6150 pounds, and we are milking 88 cows, so each of our cows is milking
6150/88 = 72.6 pounds per cow.
72.6 / 8 = 9.075 gallons per cow per day
Remember, that each cow gets milked twice each day, so that means they each give ~36 pounds of milk or 4 1/2 gallons every morning & every night.
Brent can tell you how many Dairy Queen Blizzards that is a day.
If our cows would maintain that milk production year round, they would produce (73*365) 26,645 pounds (3,330 gallons) in a year, and the entire herd, based on having 90 cows in the herd (we are a little low on cows right now) is (73*365*90) 2,398,050 pounds of milk (or 299,756 gallons) produced from our farm each year.
Now, we don't produce that much milk right now for a couple of reasons. One, our cows are doing really well right now. It's cool temperatures, and they are enjoying their new barn. We also have a lot of cows that are fresh (or just calved) this fall, and they tend to give more milk for the first 3-4 months after calving. The other reason is that cows don't milk 365 days a year, if everything works correctly. A cow should have a calf about every year. Before she has her next calf, we stop milking her for about 2 months. That gives the cow a chance to "reset" or "reboot", and put all of her energy needs towards her pregnancy. We also move them to a different pen (appropriately named "Maternity") that we bed with straw, so that the calf can be born into a dry place, without all the other cows around to potentially harm them.
Besides knowing the bulk tank weight (the weight the milk truck driver writes down), we also know how much each individual cow is milking. We have a milk tester come out to our farm monthly. Our tester, Jenny, brings milk meters that are attached to the milking machines, and give us a reading of what each cow is milking.
That's enough math for a Sunday night I think!