What are the response frequencies of sensory neurons?
The auditory brainstem shows "phase-locking" typically up to 1-3Khz at most; 3000Hz is an incredibly high firing rate for a single neuron, but this phase-locking is achieved not by individual cells firing in-phase with an auditory stimulus, but rather with a population of cells that tend to fire in-phase, such that if you average across the population you get a phase-locked population volley.

In some cases, in some animals, this phase locking can even get to the higher frequencies (see here for example).

However, this phase locking seems primarily important for sound localization via interaural time differences. Frequency itself is encoded by which population of hair cells is activated, according to the properties of the basilar membrane. Firing rates of individual spiral ganglion cells are only faster than 100 Hz at very high stimulus intensities.

Similar to the spiral ganglion cells, retinal ganglion cells primary encode intensity information in their firing rates.

However, in both cases, it's important to recognize how crucial adaptation is in sensory systems. RGCs in particular fire primarily to transients, so it is typical to use light flashes, drifting gratings, or other dynamic stimuli. The response to a "medium long, medium strong signal of some fixed (wavelength)" is going to be brief, followed by silence, not a constant response like you imply.