The Nd magnet, Nd2Fe14B, is an important material because of its high coercivity applied in modern technologies. However, the microscopic mechanism of the coercivity has not been well understood. We study the magnetization reversal of a single grain of the magnet at a finite temperature by a real-time stochastic Landau-Lifshitz-Gilbert simulation of an atomistic model, which enables us to analyze dynamical properties reflecting the atomic-scale magnetic structure. There exist difficulties to estimate long relaxation times of the reversal quantitatively, i.e., the limitation of simulation time and also dependence on the damping factor alpha. Here we develop a statistical method to estimate precisely long relaxation times in the stochastic region, by which one can identify an initial transient process and a long-time regular relaxation process. The relaxation time is found to largely depend on alpha especially in the stochastic region. However, it is found that a sharp increase of the relaxation time with lowering an external magnetic field causes a close location of the threshold fields for different values of alpha. By making use of this fact, we quantitatively estimate the coercive field at which the relaxation time is 1 s.