Use the -t option. From man ffmpeg:

-t duration (input/output)

When used as an input option (before -i), limit the duration of data read from the input file.

When used as an output option (before an output filename), stop
writing the output after its duration reaches duration.

duration must be a time duration specification, see the Time duration
section in the ffmpeg-utils(1) manual.

-to and -t are mutually exclusive and -t has priority.

Example that records for one hour:

ffmpeg -f v4l2 -i /dev/video0 -t 01:00:00 output

Or in seconds:

ffmpeg -f v4l2 -i /dev/video0 -t 3600 output