It will take 54 days for iodine-131 and 193 years for strontium-90 to reach 1% of their initial values.
Once again, your tool for solving this problem is the nuclear half-life equation
In both cases, you can write the amount left after the passing of
Plug this into the equation to solve for
Taking the base 1/2 log from both sides will yield
In the case of iodine-131, the value for its half-life is 8.1 days, which means
Rounded to two sig figs, the number of sig figs in 8.1, the answer will be
In the case of strontium-90, the value for its half-life is 29.1 years, which means
Rounded to three sig figs, you'll have
As you can see, the strontium-90 isotope poses far more long term dangers; in fact, traces of strontium-90 can be found in the teeth of all the people born after 1963 because of the extensive nuclear testing done before that date.
Read more about the health impact of these isotopes here: