Welsome to the forum XxxOoo
I suggest taking off your carto from your battery everytime you top off or fill. You don't want to over-fill and have juice running all over your battery.
If you're using a smilomizer I personally top off once per day. But I am not too heavy of a vaper. You may need to do it twice. You mentioned buying tanks, but said how often should you top off the carto. With tanks after they have been properly filled and the carto has been primed.. (I use dry hits for this, just until I can see slight moisture forming on the top of the polyfill) You don't top off the carto, you simply top off the tank section. Just don't let your e-liquid get below the 2 suction holes located towards the bottom of the carto and you should be fine.