There is no lower limit on the frequency of electromagnetic fields. One can consider a DC field as the lowest frequency being of zero Hz. Perhaps somebody would argue that to be truly of zero Hz the DC field have to exist for all eternity. However, if one expands the DC field that exists for only a finite duration with a Fourier transform, one obtains a spectrum that is continuous and nonzero at the origin. This shows that nonzero frequencies exist all the way down to zero.
What about the quantum mechanical view of this? As pointed out by tparker, one can get low frequency "soft" photons that contribute to the dynamics. Moreover, DC fields such as the Coulomb field around charged particles are also included in quantum mechanics (or quantum field theory).
One view that is expressed is that the lowest frequency should be determined by some cavity effect due to the size of the universe. However, such a view has a few practical problems. For the universe to act as a cavity, the field must be able to bounce back and forth in it so that it can constructively interfere with itself. This assumes particular boundary conditions. It also assumes that the field will exist for long enough to build up this constructive interference and obviously it must still be coherent with itself for something like that to work correctly. Just thinking about this a little bit, one quickly realized that such requirements are unlikely to be satisfied. Hence, the size of the universe probably does not set a limit to the lowest allowed frequency.