According to our current understanding of physics, light is believed to always travel at its maximum speed, which is approximately 299,792,458 meters per second (or about 186,282 miles per second) in a vacuum. This speed is often denoted by the symbol 'c' and is considered a fundamental constant in the theory of relativity.
From the moment light is emitted or created, it begins traveling at this maximum speed. There is no acceleration period for light; it attains its full speed instantaneously. This means that as soon as light is generated, it propagates at the speed of light in a vacuum.
However, it's important to note that light can experience a change in speed when it passes through a medium other than a vacuum, such as air, water, or glass. In such cases, light can be slowed down due to interactions with atoms and molecules in the medium, leading to phenomena like refraction. However, in a vacuum, light always travels at its maximum speed from the moment of its creation.
So, to answer your question directly, light takes no time to attain its full speed because it starts traveling at its maximum speed as soon as it is generated.