Short answer. By the legatee's needs and the estate. Article 944 says that if a will leaves a legacy for support without fixing the amount, it is set by the legatee's social standing and circumstances and the value of the estate. A support the testator regularly gave in life is deemed the amount, unless disproportionate.
What the law says
If the testator has not fixed the amount of such legacies, it shall be fixed in accordance with the social standing and the circumstances of the legatee and the value of the estate.
Civil Code, Article 944 — A Legacy for Education or Support. Read the full provision →
Legacies for support and for education
Article 944 deals with legacies meant to provide for a person over time rather than to give a fixed thing. A legacy for education lasts until the legatee comes of age, or beyond majority so that he may finish a professional, vocational, or general course, provided he pursues it diligently. A legacy for support lasts during the legatee's lifetime, unless the testator provided otherwise. These are continuing legacies, and the natural question is how much they are worth when the will that creates them does not state a figure.
Fixing the amount when the will is silent
The article supplies the measure. It provides that if the testator has not fixed the amount of such legacies, it shall be fixed in accordance with the social standing and the circumstances of the legatee and the value of the estate. So the amount is not left to guesswork or to the legatee's demand; it is set by weighing what the legatee's station and situation reasonably require against what the estate can bear. This keeps a support legacy realistic, neither starving the legatee nor draining an estate beyond what the testator, leaving the amount open, could sensibly have intended.
When the testator was already giving support
The article adds a practical guide drawn from the testator's own conduct. If the testator, during his lifetime, used to give the legatee a certain sum of money or other things by way of support, that same amount is deemed bequeathed. The testator's established pattern of support becomes the presumed measure of the legacy, since it shows what he himself considered appropriate. This anchors the amount in real, demonstrated practice rather than in abstract estimation, giving effect to the level of support the testator actually provided while alive.
The limit of proportionality
That presumption is not absolute. The amount the testator used to give is deemed bequeathed unless it is markedly disproportionate to the value of the estate. If honouring the prior level of support would be plainly out of scale with what the estate can sustain, the presumed amount yields, and the measure returns to what is reasonable given the legatee's circumstances and the estate's value. The rule thus balances respect for the testator's own practice against the reality of the estate, so a support legacy remains fair to both the legatee and the succession as a whole.