When the levels of sodium in our bodies fall below 135mmol/L (millimoles per liter) it's called hyponatremia. This causes fluids to shift from the outside to the inside of the cells, causing them to swell. When this happens to brain cells, it can cause brain damage, coma, and even death.

There have been many cases of water intoxication reported especially in soldiers. Referencing a source I came across while researching this topic; one report described how the death of three soldiers was associated with drinking 1- - 2- liters of water in just a few hours. The symptoms of low levels of sodium (hyponatremia) can sometimes be misinterpreted for those of dehydration. One soldier was misdiagnosed as suffering from dehydration and heat stroke, died from water intoxication as a result of repeated oral hydration. 

So you might be asking yourself, how much water is enough? Water intoxication occurs when you take in more water than your kidneys can process through urine. Symptoms of too much water intake (hyponatremia) can occur from as little as 3 - 4 liters in a short amount of time. Your kidneys have the capacity to eliminate about 20 - 28 liters of water a day but cannot get rid of more than 0.8 - 1 liters per hour. 

So to play it safe try not to consume more than 1 liter per hour. 

Also, keep in mind:

1. When thirsty, drink.
2. When you're not thirsty anymore, stop.
3. During high heat and exercise, drink enough to compensate for the loss of fluids.

Info Ref: Authority Nutrition