You have one month from the date on the Decision Letter to request an MR.
You can make a Late request up to 13 months after the date, you will need to provide your reasons for the later request.
The DWP may allow you four weeks from the date of your request to provide additional information but this includes postage so in reality you probably only have three weeks.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems