Short answer. At the testator's death. Article 866 provides that the second heir acquires a right to the succession from the time of the testator's death, even if he should die before the fiduciary. Your right as second heir vests immediately at death, not later when the fiduciary hands the property over.
What the law says
The second heir shall acquire a right to the succession from the time of the testator's death, even though he should die before the fiduciary. The right of the second heir shall pass to his heirs.
Civil Code, Article 866 — The Second Heir's Right Vests at Death. Read the full provision →
The vesting moment is the testator's death, not the eventual transfer
Article 866 answers your timing question directly: the second heir shall acquire a right to the succession from the time of the testator's death. In a fideicommissary substitution, the first heir, called the fiduciary, holds and preserves the property with an obligation to eventually deliver it to you, the second heir. But your legal right to that succession does not wait for the fiduciary's eventual transfer to materialize; it vests at the moment the testator dies, well before the fiduciary's obligation to deliver is actually carried out.
Why your right survives even if you die before the fiduciary
The article goes further to address a scenario that could otherwise seem to defeat your right: even though he should die before the fiduciary. Because your right already vested at the testator's death, your own death before the fiduciary's death or before the actual transfer does not erase that right. This confirms the right is a real, present entitlement from the moment of the testator's death, not a mere expectation that only becomes real once the fiduciary eventually performs.
What happens to your right if you die early
The article then states the consequence directly: the right of the second heir shall pass to his heirs. So if you die before the fiduciary and before receiving the property, your right as second heir does not simply vanish; it transmits to your own heirs, who then stand to receive what would have come to you. This treats your right in the succession as a genuine, transmissible asset from the moment it vested at the testator's death.
What this means for planning around your interest
Because your right vests immediately upon the testator's death rather than upon the fiduciary's eventual delivery, you hold a real, existing interest in the property from that point forward, even while the fiduciary continues to hold and manage it. This matters for your own estate planning, since that vested right is something you can pass on to your own heirs if you should die before actually receiving the property from the fiduciary.