What is the frequency/wavelength of a neutrino?
Neutrinos like all other particles have can behave like both a wave and a particle.
Neutrinos were first postulated to explain missing energy and momentum in certain nuclear reactions such as beta decay. Energy and momentum are both conserved, the neutrino has the energy and momentum which is the difference between the measured values for the parent and daughter particles. For example when a proton decays into a neutron and a positron.
Each neutrino has energy and momentum depending upon the reaction which created it.
As a neutrino is a particle which has energy and momentum. This means that is can behave both like a wave and a particle.
The de Broglie equation describes the relationship between the relativistic momentum
The wavelength of a neutrino can be anything from tiny fractions of a metre to several metres. Short wavelengths correspond to high momentum neutrinos.