In principle, you lose the status of heir if you have not opted within the period of 10 years from the opening of the succession, from the date of death. You are then deemed to renounce this succession (article 780 al. 1 and 2 of the Civil Code)
However, if you had legitimate reasons to be unaware of the opening of your father's succession, the prescription only runs from the knowledge of your heir rights (article 780 al. 5 of the Civil Code).
Please note: the postponement of the starting point cannot have the effect of extending the period of extinctive prescription beyond twenty years from the day on which the right came into place (article 2232 C. civ).