If I don't miss my guess the KI tablets will last longer than the radiation. The most dangerous isotope coming from a nuclear explosion is Iodine 131, which has a half life of 8 days. All of the iodine isotopes will be absorbed in the thyroid. The KI tablets protect the thyroid. Other isotopes last longer but they will not attack the thyroid, so KI is not a benefit.
With a half life of 8 days after about three weeks the I-131 level would be almost undetectable. I would still check with a doctor. I agree with Weedygarden. Everything I have read says KI tablets are of no benefit to older people.
