How does one know when to stop milking a dairy cow?

when her milk tastes sour.

Obviously if you aren't getting any more milk out of her udder, then she is done. If you milk her every day at the same time, and make sure you have all of the milk out, she will continue to produce the same amount at each milking. If you don't get all the milk out, or do not stay at a good milking schedule, her milk supply will dwindle to nothing.

Try Not stopping for a change. I hear it always pisses off the cows when ol` farmer Brown stops after the foreplay.

after the cow kicks u in the head.

When the cow turns around and looks at you funny and goes, "Moooooo!"

when she stops giving milk.

When her udders look floppy, wrinkled and empty.