I received a retroactive payment from CPP and in December I paid Manulife insurer some of it as an offset to LTD payments . My T4 from Manulife did not reflect the amount I paid to them. They said they will send me a letter showing the repayment but have not done so yet. Are they required to provide the letter within a certain period of time?

They do have a duty to reply to you within a reasonable amount of time, but I'm not sure if there would be a specific date by which they have to send it in this circumstance. That said, if there is a deadline upcoming by which you need it, you should definitely tell them that (in writing). Also, if you've made three or four requests for it without success, you should consider escalating the matter to their complaints department/internal ombudsman (their contact information will be on the insurer's website).