  The spin relaxation time in solids is determined by several competing energy
scales and processes and distinct methods are called for to analyze the various
regimes. We present a stochastic model for the spin dynamics in solids which is
equivalent to solving the spin Boltzmann equation and takes the relevant
processes into account on equal footing. The calculations reveal yet unknown
parts of the spin-relaxation phase diagram, where strong spin-dephasing occurs
in addition to spin-relaxation. Spin-relaxation times are obtained for this
regime by introducing the numerical Loschmidt echo. This allows us to construct
a generic approximate formula for the spin-relaxation time, $\tau_{\text{s}}$,
for the entire phase diagram, involving the quasiparticle scattering rate,
$\Gamma$, spin-orbit coupling strength, $\mathcal{L}$, and a magnetic term,
$\Delta_{\text{Z}}$ due to the Zeeman effect. The generic expression reads as
$\hbar/\tau_{\text{s}}\approx \Gamma\cdot \mathcal{L}^2
/(\Gamma^2+\mathcal{L}^2+\Delta_{\text{Z}}^2)$.
