AppleScript: start/stop QuickTime Player screen recording on timer
It's not pretty, but if you click to start the video this will stop it.

tell application "QuickTime Player"
    new screen recording
end tell

--Pause for 10 seconds
delay 10

--Click the stop recording button in the top menu bar.
tell application "System Events"
    click menu bar item 1 of menu bar 1 of application process "screencaptureui"
end tell


Pro tip: Record yourself doing the action you want to automate in automator, drag and drop that operation into a new window and copy the line where it says click menu... (Remove the "\" escape characters) into a simple tell application "System Events" command. But unfortunately it doesn't work for everything.