The longest wavelength light that can be seen with the human eye is typically considered to be around 700 nanometers (nm). This corresponds to the color red at the edge of the visible spectrum. However, it's important to note that the exact cutoff for the longest wavelength can vary among individuals. Some people may have a slightly broader range of sensitivity, allowing them to perceive slightly longer wavelengths, while others may have a narrower range. Nonetheless, for general purposes, 700 nm is often used as a reference for the longest visible wavelength.