We show that the frequency dependence of the effective critical current, resulting from thermally activated flux motion, may be used to evaluates the jump attempt frequency fo of over vortices over their pinning potential wells. We apply this analysis to estimate fo for a Nd1.85Ce0.15CuO4−y single crystal. The obtained value, fo≈109Hz, is in agreement with the theoretical estimates; furthermore, this high value, together with the experimental activation energies U(T,H,J), allow to explain the important thermally activated flux motion effects in these superconductors, observed at low temperatures or high frequencies.