If you're on a very low sodium diet and/or are on any medications or supplements, especially diuretics/high blood pressure meds and some others, you should talk to your doctor if you're drinking more than 3 quarts/liters per day.
My mom (in good health except for arthritis and mild high blood pressure) was hospitalized with water intoxication, also called water poisoning, water overdose, and hyponatremia (dangerously low blood sodium - water overdose can wash out any of the electrolytes out of your blood, but sodium depletion is most common).
The kidney specialist called in on Mom's case said he's seeing more and more cases among healthy people, usually women dieters (though it is still rare). It used to be, he said, that he'd only seen it in extreme athletes, mentally ill folks with water drinking compulsion, and drug users drinking tons of water to try to pass a drug screen.
Now he says he's seeing it more often in a wider range of people, he believes because of dieting and dieting water myths (such as the myth that other liquids don't count as water, and the myth that a person needs one half ounce per pound of body weight, for very obese people that can be very risky, especially if combined with belief in the previous myth, because obese people do not need much, if any, more water than thin folk).
My mom was only drinking a little over a gallon of liquids per day (from coffee, milk, and water), but she was also eating a low-sodium diet (our family always has), and she was on a low-dose blood pressure medication that was a diuretic.
The kidney specialist said that the sodium depletion of water overdose can occur suddenly (drinking a huge amount in a short amount of time) or gradually (just a little bit of extra water over a longer period of time).
Most people are getting way too much salt in their diet for it ever to be a problem, but if you are on medications, work out intensely, and/or are eating a very low-sodium diet, it's something you should discuss with your doctor.
I'm at risk because I'm on the same class of blood pressure medications as my mother, and like her I eat relatively little sodium, and tend towards low blood sodium levels to the point I've had to take sodium supplements before surgery (because low-sodium levels can cause cardiac arrest).
My doctor recommended a 3-liter maximum for me (I do often drink more, but not on a regular basis, and I tend to try to use a little more salt on those days).
Because of my mom's situation (a very close call, because even the emergency room doctors weren't looking for water intoxication. Thankfully they took an EKG and were investigating the abnormal heart rhythm).
It was such a close call that she was in the hospital for over a week to get her electrolytes up to a safe level to discharge her.
The scariest part of water intoxication is that by the time you are having symptoms, your life is already in jeopardy. And the symptom are so vague that they could be misinterpreted as anything from heat stroke and food poisoning to flu.
My Mom thought she had the flu, and my dad a former EMT thought so too, but he was concerned because of the odd symptom of mental confusion. He thought she might have had a stroke. Thank God for that symptom, or she probably would have died of a heart attack at home.
Don't get me wrong, I'm not saying it's common, just possible. With the number of otherwise healthy people on diuretics and blood pressure medications, it really should be something that patients are warned of when put on those medications, but I guess because most of us are getting 5 to 10 times the sodium we need, I guess doctors still think it's too rare to mention.