Short answer. Half of it. Article 900 gives a sole surviving spouse one-half of the estate as her legitime and leaves the other half to your free disposal. One narrow exception cuts her share to one-third: a marriage celebrated in articulo mortis where you die within three months of it.
What the law says
If the only survivor is the widow or widower, she or he shall be entitled to one-half of the hereditary estate of the deceased spouse, and the testator may freely dispose of the other half.
Civil Code, Article 900 — Legitime Of Sole Surviving Spouse. Read the full provision →
The half you can direct, and the half you cannot
The rule is symmetrical: your wife's legitime is one-half and the testator may freely dispose of the other half. That free half can go anywhere — a sibling, a godchild, a charity, a friend, or to your wife as well if you want her to take more than the minimum. What you cannot do is reduce her below the half, whether by giving it away in the will or by disposing of it in lifetime donations that leave the estate too thin to satisfy her. A legitime is a floor, and dispositions that breach it are reduced.
The deathbed-marriage exception
The second paragraph is easy to miss and occasionally decisive. If the marriage was solemnised in articulo mortis — at the point of death — and the testator dies within three months of it, the sole surviving spouse's legitime drops to one-third rather than one-half. The provision exists to blunt the obvious abuse: a marriage contracted in a hospital bed for the sake of the estate. It is not aimed at couples with a genuine history together, which is why it comes with the qualification in the sentence that follows it.
Five years of living together restores the half
The exception is itself excepted when they have been living as husband and wife for more than five years. Where that is true, the legitime returns to one-half however close to death the ceremony took place. So a couple who lived together for a decade and married only when one of them fell ill are treated as an ordinary married couple for this purpose — the timing of the wedding costs them nothing. Both conditions are factual, and both are provable or disprovable by evidence rather than by argument about intention.
Check that she really is your only compulsory heir
The whole article rests on the phrase if the only survivor is the widow or widower. Living parents are compulsory heirs, and so is any child, legitimate or illegitimate, acknowledged or not yet acknowledged — and any of them changes the fractions entirely. A will drafted on the assumption that a spouse stands alone, when a parent or a child in fact survives, is the classic setup for a claim after death. Before executing anything, list every ascendant and descendant who might outlive you and have the will drafted against that list.