Now half live is a quite general "property", the half life is related to the matrix interaction element between final and initial state, which makes both the states and the interaction itself subject to investigations and definition.
For example the decay rate (half life) of a nucleus is given by the same formal "equation" as the decay rate for a baryon, or an elementary particle (such as the muon).
So by asking if string theory have any equation for calculating the half life of let's say the isotope Sn-100, is similar to ask if QCD provides it. Both theories are not for describing larger systems, where the internal degrees of freedom not matter (i.e they are not effective field theory of nuclear structure physics), so one needs something else.
Now String Theory, as far as I know, don't provide any decay rate calculations on elementary particle decays. At least not any which is not predicted according the "standard model" or the supersymmetric models, i.e the effective field theories to string theory..