The end of the Grand Unification Epoch

Question

In standard textbooks of cosmology it is sometimes mentioned that the Grand Unification Epoch ended at approximately  $t_{\small GUT}\approx 10^{−36}$s after the Big Bang. At this point, the strong force separated from the other fundamental forces. Is there a (simple) formal derivation to get this estimate?  

EDIT: Maybe an answere can be found by reasoning why the grand unification energy is taken to be $10^{15}$ GeV or so.

Comments

I guess there are ones, but they really depent on the initial hypothesises.

For example, slow neutrons in the neutron waveguides are a manifestation of strong forces too, but the time and space scales are there quite different from BB conditions.