Can you explain better?

Can't the wave length of a single photon be found by dividing the frequency by the speed of light? That should give you one wave cycle. Since a quantum is the energy-time of a single photon, and the time portion is linked directly to frequency, it seems the time duration of a quantum needs to be one wave cycle.

Here's a Wiki take on it

A photon is often referred to as a "light quantum". The energy of an electron bound to an atom (at rest) is said to be quantized, which results in the stability of atoms, and of matter in general. But these terms can be a little misleading, because what is quantized is this Planck's constant quantity whose units can be viewed as either energy multiplied by time or momentum multiplied by distance.

In support of your idea I have this little consideration (which I'm sure you already had): the phase φ of a wavefunction is, in the classical limit, S/

~~h~~ where S is the action, so S =

~~h~~φ; the minimum value of the action, since

~~h~~ is constant, comes for the minimum of the phase φ, which is 2πn where n is the number of cycles; the minimum of n is 1 so if we assume that there can't be a fractionary number of cycles (that is, that n must be an integer) to have an inter-action between two bodies, then we have that the quantization of the action could be interpreted as the fact we must have an integer number of cycles, for an interaction (more precisely, one, at least).

But the fact a single photon could be associated to a single cycle of EM radiation, fights against the fact that the frequency of an EM radiation can be more or less determined (ex. spectral lines more or less sharp) and with the fact that the emission time of an atomic (for example) transition can vary, and a lot. How can you explain these two facts if you assume that a photon is 1 single cycle of EM rad.?