That really depends on a few things what type of juice your using cause some juice especially custards really can turn your coil and wick black after 2-3 tanks how hot your building your coils and the diameter of your coil because the smaller the wick the easier it will collapse. Personally I never tried to do any test because wicking material even kgd is really cheap IMO so there really isn't any reason to not rewick but honestly if I'm vaping a high vg juice that has been heavily sweetened then it will gunk your coil up in like 2-3 tanks from there I pull my wicks out and I give my coils a dry burn and rewick it so I would say 2-3 tanks is when I feel it isn't performing great you could probably keep going on that same wick and coil for a while 1-2 weeks easy but like I said above there's no reason not to rewick.
Don't mind top fill, just the crap tank, could of easily be 2 mm taller and with an SS tank section, an would of been sweet if it came with a Allen wrench, heck the Big Dripper came with 2 allen wrenches, and a full set of back up screws, O-rings, and such. And didn't come in a tp cardboard roll, though Fae and Bea enjoyed chewing on that, so not all was a loss