A wave with an infinitely long period isn’t really recognizable as a wave. It’d just be interpreted as a flat line anywhere in the universe. And as mentioned, the energy of light is tied to its frequency: E = hf. (Or with hbar • omega, but that’s just multiplied with and divided by 2π, so, the same thing.)

So an infinitely long wave would have f=0 and thus no energy.

The highest frequency you’d get would be 1/planck-time, so the energy would be the Planck constant divided by Planck time, which would be roughly 12.3 GJ. That’s a lot of energy for just one photon, but if it’s just the one, likely not world-ending.