There is no need to take it at all unless you are warned by the government that radiation is about to hit. You would then need to take it within the hour of radiation fallout for maximum effect. It is not advised to take it 'just in case' It is useless in that case.
I would not advise it, due to the fact that it contains Iodine, It does not protect the rest of your body from radiation anyway, it will only fill your thyroid with Iodine and prevent it from absoring radiation, but not anything else. in addition may cause problems with your meds. FTB4