Short answer. Up to one-fifth of their present property, and only where they have chosen a regime other than absolute community. Anything above that fraction is void as to the excess. Donations of future property are a different matter and follow the rules on wills.
What the law says
If the future spouses agree upon a regime other than the absolute community of property, they cannot donate to each other in their marriage settlements more than one-fifth of their present property. Any excess shall be considered void.
Family Code, Article 84 — Limit on Donations Between Future Spouses; Future Property. Read the full provision →
The cap only bites under some regimes
Read the opening condition first: the limit applies if the future spouses agree upon a regime other than the absolute community of property. Under absolute community the ceiling is beside the point, because almost everything the spouses own is pooled anyway and a donation between them moves nothing. It is the couple who have chosen conjugal partnership or complete separation — the couple, in other words, who deliberately kept their patrimonies apart — whose gifts to each other in the settlement are capped. The rule exists to stop a settlement that separates property in form from transferring it wholesale in substance.
One-fifth of what, exactly
The fraction is measured against present property: what the donor actually owns when the settlement is executed, not what they expect to own, and not the couple's combined holdings. Each spouse's fifth is computed on that spouse's own patrimony, so a wealthy party and a modest one do not share a single allowance. The valuation date follows from the same word — present, at execution — which is why a settlement that gives a percentage rather than a named asset is easier to defend later than one that quietly exceeds the cap because a property was worth more than anyone thought.
Only the excess falls
The sanction is deliberately narrow: any excess shall be considered void, not the whole donation and certainly not the settlement. A donation of half the donor's present property is good up to one-fifth and void beyond it. That is a real comfort to a couple who miscalculated, but it is not a reason to be careless, because the reduction is worked out years later by people with an interest in a different answer — heirs, usually, or a creditor. Better to fix the figure at the outset than to leave a court to trim it.
Future property is a different instrument
The last sentence quietly changes subject. Donations of property the donor does not yet own are governed by the provisions on testamentary succession and the formalities of wills. A promise to give what you will one day acquire is, in effect, a legacy, and it has to be executed with a will's formalities to be worth anything — witnesses and all. A clause buried in a notarised settlement will not do. If what you have in mind is providing for each other out of everything you will accumulate, the document you need is a will, drafted as one.