Great question...we've gone back and forth on this topic here in the office.It's really hard to determine this number because water quality plays such a huge role. Our filters have a really small micron size. So, the filter will typically either slow down or completely clog once it's reached the end of it's life. That being said, a new filter will always work better than an old one. So, we recommend replacing it every 3-4 months depending on how much you use it.If we had to give you a number, we would estimate 150 gallons (565 liters).FYI...you can save 20% on replacement filters by signing up for our Clean Water Club.