A camera using an autofocussing system is 3 m away from the subject of the photograph.?

Question

How long is the transit time of the light pulse from the camera to the object?

Answer 1

Light travels at 299.792.458 m/s.
(It's not just a good idea, it's the law.)
So a light pulse covers 3 meters in
3 / 299.792.458 = 0.0000000010007 seconds.

Answer 2

you are aware that an auto focus camera does not use a light pulse. some cameras use a high pitch sound pulse and measures the return time similar to the echo location used by flying bats. Also objects do a better job of reflecting sound than they do light, you would not be able to focus on someone wearing a black suit. Digital cameras often work by measuring the maximum "sharpness" of the image. they do this by moving the lens, a mechanical process which is why they may take several seconds to achieve focus.