v4l2-ctl - set video length in seconds

Question

How I can set video length?
In my example

v4l2-ctl --verbose -d 0 --set-fmt-video=width=1280,height=720,pixelformat=MJPG --stream-mmap --set-parm 25 --stream-to=video.yuvj422p --stream-count=100

I can set max framerate

--set-parm 25 

I can set frames amount.

--stream-count=100

And it will not works... Because camera can decide change my framerate to less value because of not enough light. For dark environment camera can decide not 25 fps but 7.5 fps. But I want 15 seconds video. How set video length in seconds?