The longest possible length of a light wave corresponds to the lowest frequency in the electromagnetic spectrum. Electromagnetic waves can have extremely long wavelengths, and the longest known wavelengths are associated with radio waves.
Radio waves have wavelengths ranging from several centimeters to kilometers or even longer. The lower frequency limit for radio waves is determined by the physical size of the source and the receiver. For example, for a conventional antenna, the size of the antenna determines the practical lower limit for the radio waves it can detect.
Beyond radio waves, there are even longer wavelengths associated with extremely low-frequency (ELF) waves and extremely low-frequency magnetic fields, but these are not typically considered as light waves.
It's worth noting that as the wavelength of light waves increases, their frequency decreases. The relationship between wavelength (λ), frequency (f), and the speed of light (c) is given by the equation c = λf, where c is approximately 3 x 10^8 meters per second.