not sure when it becomes a problem. lots of info says if you have pulsating zenia that you should keep your iodine levels high, especially when fragging, but i havent seen any evidence that it matters. i guess the zenia use iodine more than other corals.
i dosed my tank for about a 1 1/2 month and actually noticed a decline in health. not sure if it were due to the iodine or not. the level was at around .8.
sorry im not much help.