Hi, it is unlikely that someone in a diabetic coma would be unconscious for this length of time as once the imbalance in blood sugar & electrolytes has been corrected the patient usually regains consciousness. Unless of course they developed brain damage as a result in which case it's unlikely they would make a full recovery. If you want to base your play on someone who is in a coma for this long & recover it may be more credible to base it on someone who has sustained trauma to the head I.e. brain injury as there have been cases of patients who have regained consciousness after lengthy periods in a coma & made a relatively full recovery etc.

Hope this helps