Is there a deadline I need to reclaim by?
No. Though there are some guidelines which help. It's a lot easier if your insurance was active in the last six years.
Insurance started in the last six years: There's no issue here at all. Even if the loan's now paid off, you can start a reclaim.
Older insurance that's still active, or ended within the last six years: You can start a reclaim. The six year rule applies to active insurance, so a policy taken out 12 years ago but paid off five years ago was still active within the key six year period.
If your policy ended over six years ago: If this is the case, the ‘statue of limitations’ means banks don’t need to keep records. Yet the Ombudsman doesn't officially have a cut-off time. So if you’ve still got the paperwork, while your chances of success are a little lower with older loans, many still do successfully reclaim.