More precisely, it would become a constant electric and magnetic field.

Alberto - can you explain that, please?

Let's consider electric field only, for simplicity, and in a specific instant of time.

If the wavelenght is, let's say, one metre, it means that in one metre the electric field varies from zero at the initial point, to its maximum positive, e.g., 1 (25 cm away the initial point) to zero again (50 cm away) then to -1 (75 cm away) then to zero again at the final point 1 metre away.

If however we limit our analysis to what happens between two points separated by 1 mm only, you understand that the electric field doesn't vary much between them: if they are near E = 0, then the field will vary let's say from 0 to 0.01, if they are near 0.8 the will vary, e.g., from 0.8 to 0.78 ecc. The same if you take a wavelenght of 1 km and you consider two points 1 metre apart, or a wavelenght of 1000 km and two points 1 km apart ecc. If you imagine an almost infinite wavelenght, you understand that you will need billions of light years of distance to find a slight variation in E field.

If we instead stay in the same point and we want to analyse variations in time, we can have that previous variation letting light arrive: it travels at c so you have to wait billions of years to have that slight variation; in other terms, we can compute the variation in time, fixed the point in space, simply by dividing that space separation by light's speed c.

In formulas:

ν = c/λ

ν = frequency; λ = wavelenght.

If λ goes to infinite, frequency goes to zero; but frequency is the number of cycles per second in a specific point. If this number is almost zero, it means the field is almost non varying; when ν = 0 the field is constant.