Similarly the reverse is possible where a photon can spontaneously morph into two equal particles of matter and antimatter.
Wrong, a photon has to interact with some other particle or field in order, if it has enough energy , to create a particle antiparticle pair.
This is because of special relativity: the photon has a mass zero whereas the particle antiparticle pair will have at least twice the mass of the particle , i.e. the dot product of the two generated four vectors should have that mass. Momentum conservation would be violated, since in their center of mass the pair would have momentum zero , all the energy would go to the pair masses and opposite to each other momenta adding to zero, whereas the photon carries momentum h/lamda in all frames .
The probability of a gamma generating a pair of particle antiparticle can be calculated using the appropriate to the target fields Feynman diagrams. For example, the left diagram of scattering off the field of an atom Z:
pair production by a gamma ray (left) or an electron (right).
There is a prescription that turns a Feynman diagram into an integral, and the evaluation of the integral will give the probability of this reaction happening (how often in your title)