First off, how much water is excessive for a dog?
The average dog needs 20-70 ml per pound of their body weight of water a day (example/ My 150 lb malamute requires 3,000 to 10,500 ml of water a day). To give you a rough idea of how much that is one cup equates to around 237 ml. (In my example that would be 13-40 cups of water!) I would increase this amount on hot summer day or after rigorous exercise.
What causes excessive thirst in dogs?
-Kidney Failure
-High blood calcium
-Uterine infection
-Diabetes mellitus
-Abnormality of the pituitary gland
-Kidney or Kidney tubules abnormalities or defects
-Kidney infection
-Over or Under activity of the adrenal glands
-Urinary tract blockage or defect
-Electrolyte imbalance
-Liver disease
-Behavioral problems causing excessive drinking even when not thirsty
-Heat
-Excessive activity
-Miscellaneous disorders (that's helpful)
As you can see the list of causes for excessive thirst in dogs is rather extensive and varied, it could be you have an ornery dog, a dog whose just far too hot or your dog's kidney's could be failing (kidney failure is one of the main causes of Polydipsia) It's hard to say based just on excessive thirst.
The best way to determine the cause of excessive thirst in your dog is to first monitor the dog for a day or two. See if the condition doesn't change with environmental differences (heat), attention (more common in puppies) or time. If it doesn't you should take your dog to the vet for further testing to ensure the cause is not physical. In the mean-time, keep the toilet seat down.