Reaching the speed of light is not possible for objects with mass, according to our current understanding of physics. As an object with mass approaches the speed of light, its mass would increase, and it would require an infinite amount of energy to reach or exceed the speed of light. Therefore, it is not feasible to reach the speed of light in a spaceship.
However, if we consider accelerating to a significant fraction of the speed of light, we can discuss the effects of acceleration on the human body. The main concern during high-speed acceleration is the g-force experienced by the human body, which is the force equivalent to the acceleration due to gravity.
The human body can withstand a range of g-forces depending on the duration and direction of the acceleration. Brief exposure to high g-forces can be tolerated, while prolonged exposure can be harmful or even fatal. The exact tolerance varies between individuals, but typically, humans can withstand accelerations up to about 5 g for a limited time without suffering severe adverse effects.
To minimize the potential harm from high g-forces, spacecraft designed for high-speed travel may use gradual acceleration over an extended period. By gradually increasing the acceleration, the human body can adjust to the changing forces and reduce the risk of injury.
The specific time required to reach a certain velocity while minimizing the g-forces experienced would depend on several factors, including the level of acceleration, the distance to be covered, and the specific spacecraft design. Calculating the exact time would require detailed knowledge of the spacecraft's acceleration profile and the limitations imposed by human physiology.
In summary, reaching the speed of light is not possible for objects with mass. If we consider reaching a significant fraction of the speed of light, the time required to accelerate without harming a human inside would depend on various factors and would require a detailed analysis of the specific spacecraft design and acceleration profile.