A recently observed phenomenon of the time-dependent deformation of ferromagnetic Ni-Mn-Ga martensites under a stationary magnetic field has been studied. A statistical model describing this phenomenon has been deduced from the mathematical theory of random processes. The influence of the duration of measurement on the shape of strain-field curves has been observed experimentally and interpreted in the statistical model framework of the time-dependent deformation. The evolution of deformation of the Ni-Mn-Ga alloy in the stepwise magnetic field has been considered. Good agreement between the theoretical and experimental results has been achieved for the suitable set of the model parameters.