Short answer. It binds until it is annulled. A contract agreed to in a state of drunkenness is voidable, not void — it produces effects and must be obeyed unless and until the person whose consent was impaired brings an action to annul it and a court sets it aside within the period the law allows.
What the law says
Contracts agreed to in a state of drunkenness or during a hypnotic spell are voidable.
Civil Code, Article 1328 — Lucid Intervals; Drunkenness; Hypnosis. Read the full provision →
Voidable is not the same as void
The distinction decides everything that follows. A void contract never existed in law and anyone may treat it as a nullity. A voidable contract is valid and enforceable on its face; it simply carries a defect that one side may use to attack it. Until that side sues and succeeds, the contract stands, the other party may demand performance, and third persons may acquire rights under it. Only the party whose consent was defective can attack it — the sober counterparty cannot escape a bargain by pointing at the other's condition. And the right can be lost by ratification, expressly or by conduct such as accepting the benefits once sober.
How drunk is drunk enough
The article is not a rescue for anyone who had a few drinks and now regrets the deal. It groups drunkenness with a hypnotic spell for a reason: both are treated as states in which the person cannot give real consent. What is required is intoxication that deprived the signer of reason and of understanding of the act — not mere loosened judgment or lowered caution. Ordinary buyer's remorse after a boozy dinner does not annul anything. The same article confirms the opposite case: contracts entered into during a lucid interval are valid, so someone who is usually incapacitated but who understood the transaction at the moment of signing is bound by it.
Proving it, and the clock
The burden falls on the person alleging the defect, and it is a demanding one because a signature carries weight. Useful evidence includes medical or hospital records, receipts and bills showing what was consumed and when, photographs or video from the occasion, messages sent that night, testimony of those present, and the sheer improbability of the bargain's terms. The action to annul is subject to a prescriptive period counted from the time the incapacity ceased, so a person who sobers up, discovers what he signed and then does nothing for years may lose the remedy entirely. Delay also looks like ratification, which independently cures the defect.
What annulment actually costs
Annulment unwinds the contract; it does not simply free you from it. On annulment the parties are ordinarily required to restore to each other what they received, with fruits, interest or their value — so money spent or property already transferred must be accounted for. Where restoration has become impossible through your own act, the remedy may fail. Rights that innocent third persons acquired in good faith before annulment can also survive. In practice this means acting quickly and in writing: repudiate the contract as soon as you learn of it, avoid accepting any further benefit under it, and book a consultation before the deadline runs.