Do you have the formula for radio-active decay either in function form or in DE form?
You have to find the time it takes where the quantity is halved.
So if you set up f(t0) = e^(-kt0) and f(t1) = e^(-kt1) then f(t1) = 1/2*f(t0) for some t1 > t0.
We get 1/2 = e^(-k[t1-t0]). So we get -ln(2) = -k*[t1-t0] which means our half life time is t1-t0 = ln(2)/k.
This is the time it takes for your quantity to reduce in half.