The time scale in coherent resonant tunneling is investigated by calculating the Ohmic conductance versus frequency, in one-dimensional single-electron terms, for a resonant-tunneling diode. When the Fermi level is at the resonance energy, the conductance rolls off near the frequency equal to (resonance width)/(Planck’s constant). However, for a non-symmetrical diode structure it first rises to a sharp peak. When the Fermi level is displaced from the resonance energy, there is a conductance peak before roll off for both symmetical and non-symmetrical structures, with a peak frequency then approximately equivalent to the Fermi level displacement. At high frequencies, all cases coincide in a universal curve. For the overall three-dimensional diode, in the symmetrical case there is no frequency dependence on the resonance-width scale. The effect of the screening due to the space charge is investigated.