We consider self-annihilation of fermionic sub-GeV dark matter into metastable mediators inside white dwarfs (WDs). We assume these mediators decay into two photons after being produced. We calculate the luminosity of WDs due to these annihilations taking into account theenergy loss of the mediator and its finite lifetime in the dense stellar medium. Finally, using complementary sets of measurements ranging from cold white dwarfs in the M4 globular cluster and direct/indirect searches we discuss constraints on mediator lifetimes.