Disclaimer: I do not recommend people to do what I do. I do not suggest/imply what I do is the correct and legitimate. I am not saying I2 is a necessary reef tank supplement.....(I better find a hot key for all these stuff above!)
Now, let's get down to your question. IMO, Every tank is different. Some tank's I2 depletes quickly, some don't, some people don't even test and replenish for it. I have a 150G all softies/anemone tank. I find that if I follow the instruction on Kent's Lugol, the Salifert's reading is always "0". If I double the dosage for 2 weeks straight, I get the reading of .02. If I simply dose a full dropper full twice a week, I get the reading of .04. (while the suggested level is .06) I have been doing this for some years and has not had problems (with overdosing). I believe it really depend on each individual. Some people perform Water changes regularly and never dose I2, yet can maintain a lush growth of corals. MAYBE, maybe, I2 really do not need to be replenished as long as you do regular water changes. But, since I am an anal person and just have to make sure all parameters are in check, I DO test for I2 and DO dose I2. (by the way, some other brands of I2 supplement, other than lugo's, I found them to be useless. Even if I dump a whole bottle in, I still do not get a reading)
