Reading
the documentation for MixMonitor(), which A2B uses to record the calls, shows that it doesn't have a timeout parameter so it's not simple to do what you want with A2B v1.3.x.
That page also indicates that MixMonitor exposes an Asterisk CLI/AMI interface capable of stopping recording, so what you want would be fairly easy if there were already some piece of code monitoring the active channels on a regular basis. This is where asiby steps into the limelight, as he's been working on such a piece of code for a while now. Perhaps you might see your feature request in a future version of A2B one day.