I use Smoktech LR XL cartros without a tank. I usually toss a carto after vaping about 20 ml of unflavored through it. At my vaping rate that takes me a litle more than a week. I have vaped as much as 40 ml of unflavored through a carto but the taste of that last 20 ml wasn't up to par. After 20 ml or so I notice darkening of the liquid in the carto. That means the carto is accumulating old, heated liquid and the taste is beginning to suffer. I top off cartos by dripping into the top end until surplus flows from the bottom threaded end onto kleenex. It's easy to see the juice color on the kleenex.I usually go through a carto in three days. after that it either has a tight draw or the flavor is muted or both. i'm sitting here amazed I still have the same boge single coil two hole cart I started with. anyone have a good idea of how long a cart will last using unflavored? also I imagine you get similar results with a rba.