Light travels at a constant speed in a vacuum, commonly referred to as the speed of light. In a vacuum, such as outer space, light travels at approximately 299,792 kilometers per second (km/s) or about 186,282 miles per second (mi/s). This is denoted as "c" in physics.
Since light travels at a constant speed, it attains its full speed almost instantaneously. As soon as light is emitted, it begins to travel at its maximum speed of about 299,792 km/s. In practical terms, this means that as soon as light is generated or emitted from a source, it is already traveling at its maximum speed.
It's important to note that light can experience interactions, such as absorption or scattering, when passing through various mediums such as air, water, or other transparent substances. In such cases, the speed of light can be slightly slower than its maximum speed depending on the medium's refractive index.
However, in the absence of any interactions with a medium, light reaches its maximum speed as soon as it is emitted or generated.