If your review is due in Dec 21 then at the moment you can expect to be reviewed in approximately May/June 21. However, the DWP are working through the list of pensioners and you are likely to hear in due course about having a light touch review. It is impossible to say when this will be as they are going for the claimants who are due for a review soon - so it may not be until you are actually due for the review.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems