Hi Kerry,
There is an absolute limit of 13 months (from the date of the MR decision) in which to submit an appeal to the Tribunal Service.
However, for anything over 1 month you would have to explain why you were late with your request.
The PIP guide has a link to the SSCS1 form that has to accompany a request and it explains the process in detail.
If you have further questions please reply to this message and we will do our best to help