A medical treatment complaint should be made at the earliest opportunity and, at the most, within 12 months of the relevant treatment. This guideline is in place to ensure that the doctors, nurses etc recollection of events is clear and to ensure that your medical records are readily available for review.
The NHS has discretion to consider complaints outside of 12 months and will consider issues such as the duration of your illness and the date that you were first aware of concerns regarding your medical treatment.
It is always worthwhile submitting a letter of complaint, even if you are outside the time limits as they may respond.