If the firm that advised you is still trading
You generally have 6 years from the act or omission you're complaining about, or 3 years from the date you first became aware (or reasonably should have become aware) that something might be wrong — whichever is later.
If the firm has sent you a final response letter, and that letter told you you have 6 months to refer the matter to the Financial Ombudsman, then a separate 6-month window applies from the date of that letter.
Reference: FCA Handbook DISP 2.8.2R.