Actually, when the flavor starts to diminish, just add a few more drops of liquid and continue using. Depending on the juice you are using the carto should last you several days. When I first started vaping I generally got 2-3 days from a carto. I am now using a carto tank and generally get a week or more out of a carto. You will know when it's time to replace the carto when it no longer vapes well or the flavor doesn't come through right.