According to the theory of special relativity, time dilation occurs as an object's velocity approaches the speed of light. However, it's important to note that time dilation effects can be observed at any relative velocity, not just when approaching the speed of light.
The magnitude of time dilation depends on the relative velocity between two observers or reference frames. The closer an object's velocity is to the speed of light (c), the more pronounced the time dilation effects become. As the relative velocity increases, time dilation becomes more significant.
However, the specific threshold at which time dilation becomes noticeable is subjective and depends on the level of precision or sensitivity of measurement. For practical purposes, significant time dilation effects are typically observed at velocities approaching a substantial fraction of the speed of light, such as 0.9c or higher. At such velocities, time dilation can become measurable and noticeable compared to reference frames with lower relative velocities.
It's worth noting that time dilation effects are not limited to high-speed objects or situations. Even at everyday velocities, time dilation is present but is typically negligible and not easily detectable without very precise instruments.
In summary, time dilation becomes more pronounced as an object's velocity approaches the speed of light, but there is no specific threshold that universally marks the point at which time dilation begins. It is a gradual effect that becomes increasingly significant at velocities approaching the speed of light.