Use this to cut video from [start] for [duration]:

ffmpeg -ss [start] -i in.mp4 -t [duration] -c copy out.mp4

Here, the options mean the following:

-ss specifies the start time, e.g. 00:01:23.000 or 83 (in seconds)
-t specifies the duration of the clip (same format).
Instead of -t you can also supply the end time with -to.
-c copy copies the first video, audio, and subtitle bitstream from the input to the output file without re-encoding them. This won't harm the quality and make the command run within seconds.

Note that if you've used -ss, you have to subtract this from the -to timestamp. For example, if you cut with -ss 3 -i in.mp4 -to 5, the output will be five seconds long.