What is the highest possible frequency for an EM wave?

It's theorized that the Planck length is the smallest meaningful unit of distance. 

A wave with that wavelength would have a frequency of ≈6.2⋅1034Hz$\approx 6.2\cdot {10}^{34}\text{Hz}$. A gamma ray typically has a frequency of >1019Hz$>{10}^{19}\text{Hz}$.

Since the energy of a photon is directly proportional to its frequency, this theoretical upper bound would require vastly more energetic processes than those we presently conceive of. 

The individual photons involved would each be carrying
41joules$41\text{joules}$, or 2.56⋅1020eV$2.56\cdot {10}^{20}\text{eV}$, of energy.

The highest measured frequencies of EM waves are Gamma-rays and are typically produced from the decay of atomic nuclei. The most powerful sources of gamma-rays (and usually the sources with the shortest wavelength) are caused by astronomical events. Recently there was a very strong gamma-ray burst from Cygnus-A,.

It is estimated that the gamma ray burst was the result of the black hole gobbling up something with three times the mass of the Earth. There is no theoretical upper limit for the frequency of gamma-rays. To make one bigger than what we've seen so far will require starting with a super-massive black hole and something much larger than the Earth. Not quite reproducible in the laboratory.

https://physics.stackexchange.com/questions/43063/what-is-the-highest-possible-frequency-for-an-em-wave