That will depend on the coffee maker you choose because water reservoirs can have different capacities. Some machines can only make for a single cup. Others can fit enough water in the tank to deliver up to five cups of coffee. But most appliances are big enough to fill an entire 10-cup or 12-cup carafe. Some machines will come with an indicator of how much water is left in the tank. In some cases, the reservoir will be transparent so that you can monitor the water level yourself. Either way, the frequency of refilling the water reservoir will depend on how quickly it gets empty.