  We investigate the long-time stability in the neighborhood of the Cassini
state in the conservative spin-orbit problem. Starting with an expansion of the
Hamiltonian in the canonical Andoyer-Delaunay variables, we construct a
high-order Birkhoff normal form and give an estimate of the effective stability
time in the Nekhoroshev sense. By extensively using algebraic manipulations on
a computer, we explicitly apply our method to the rotation of Titan. We obtain
physical bounds of Titan's latitudinal and longitudinal librations, finding a
stability time greatly exceeding the estimated age of the Universe. In
addition, we study the dependence of the effective stability time on three
relevant physical parameters: the orbital inclination, $i$, the mean precession
of the ascending node of Titan orbit, $\dot\Omega$, and the polar moment of
inertia, $C$.
