The same formula, applied to a heavier lepton

Assumed background: the muon decays only weakly; mass dimensionsThe power of mass in which a quantity is measured once $\hbar = c = 1$, fixed by requiring the action to be dimensionless.. Fermi's theoryA theory valid only below some energy scale, which can tell you from the inside where its own description stops working. gives the muon decay rate $$\Gamma = \frac{G_F^2 m_\mu^5}{192\pi^3}.$$ The fifth power is forced: $[\Gamma] = 1$, $[G_F^2] = -4$, and once $m_e$ is dropped the muon mass is the only scale left. Every approximation has a range, though, and the useful question about a formula is where it stops working and what the failure measures.