The top reservoir for tap water is roughly 1 gallon and the bottom reservoir is 2 gallons. You can expect to filter about 1 gallon every 1 hour to 1.5 hours if you fill the top reservoir completely (the more water that is on the top, the more pressure there is = faster filtration). So roughly about 2 hours to fill up the entire bottom reservoir (2 gallons). Our filter takes a little more time because of how tight our pores are. This allows us to filter out more than other brands and assure your water is safe. WE RECOMMEND you refill the top basin regularly to make sure you always have water after each use.