I change my bubbler water as much as possible. Sometimes being stoned prevents me from finding it worthwhile to change the water. The amount of water in the bubbler is the real issue. I have around a dixie cups worth of water, that gets bowls snaped into it all the time. The impurity of the water changes much faster than a bong with a big old reseviour. I hide my bong far to much to ever get to the point where it NEEDS to be cleaned. Basically when it tastes digusting as hell though.