There is no legal right to extend the time. However, several members have reported that they were given extra time, but it's usually only a couple of weeks, and this is not consistently applied. If you are struggling to get in your response, don't wait until the last minute to ask for more time. Don't try saying that you are waiting for proof from health professionals because they won't wait for that. If you can't get your response finished - you should send what you have done in - and tell them that you were unable to do more because of the shortage of time.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems