command line - Record webcam automated with unknown duration

I need to record (not stream) from my webcam, convert it to H.264 and store it in a mp4-container. With VLC media player I can do this manually with the Open Capture Device menu where I just have to press "convert" and set destination file and video codec.

But I need to do this automated, e.g. with command line or as batch - so that I can start recording from another program - and I don't know how to do that. Second problem is that I don't know the duration beforehand so I need also a possibility to stop recording (maybe a start and stop command ?).

EDIT: I'm using Windows 7.