Cats are descended from desert animals and naturally drink little. When the water bowl starts emptying fast, the change is worth investigating. Run through this list, note what applies, and bring your notes to the appointment.
The bowl is empty by midday, when it used to last three days. Litter clumps are larger and heavier than usual, or the litter box needs scooping twice as often. Your cat visits the bowl at night, when most healthy cats sleep through. Weight is dropping despite a normal appetite, or the appetite itself has increased while weight falls. Vomiting appears more than once a week.
Any two of these together justify a same-week appointment, not because every cause is urgent, but because two of the leading causes respond far better to early treatment.
Chronic kidney disease is the most common diagnosis behind increased thirst in cats over seven. Kidneys lose concentrating ability gradually, the cat drinks more to compensate, and bloodwork tracks the decline in stages. Managed with prescription diets, fluid therapy, and medication, cats often hold a good quality of life for years after diagnosis. Diabetes is the second frequent culprit, particularly in overweight middle-aged cats, and presents with the same trio of thirst, appetite, and weight change.
One practical step before the visit: measure. Note how much water you fill the bowl with and how much remains after 24 hours, for two or three days. A number beats an impression, and it lets your vet judge severity before any tests are run.