Here is a double slit experiment with single photons at a time
In the last frame the Young interference pattern appears, so it is the same conditions of distance taken from the wavelength of the emergent light from the superposition of photons. The photons have energy E=h*nu where nu is the frequency of the light that will emerge from zillion such photons. To a frequency a wavelength can always be assigned:
On the left the footprint of photons seems random, but the probability distribution, which is the emergent interference pattern, shows that the probability amplitude of a photon being found at the (x,y) of the screen has information about the wavelength of emergent light. The modulus squared of the wave function of the photon gives the probability. How this happens needs to be studied in quantum field theory.
So it is the wavelength in both cases , of electron and photon interference, for the electron given by the de Broglie wavelength.