Hey fireblade, do I remember seeing a response to this somewhere else already?
Currently (AFAIK) there’s no way to do this for an individual Skill. You can set the listener.recording_timeout value in your Mycroft Configuration however this will only work for that device.
Have you confirmed that the last digit is cut off during recording rather than STT or number extraction?
You can set listener.save_utterances to True along with a listener.save_path to keep a copy of your recordings.