Answer
Under Article 90 of the Revised Penal Code, the crime of libel and similar offenses prescribes in one year, while oral defamation and slander by deed prescribe in six months. Once the period lapses, the offense can no longer be prosecuted.
Article 91 explains how the period is counted: it runs from the day the crime is discovered by the offended party or the authorities, is interrupted by the filing of the complaint or information, and does not run while the offender is absent from the Philippines. The prescriptive period for cyber libel has been the subject of separate litigation, so the timing of an online-libel case should be confirmed with counsel.
Sources & further reading
Research aid — not legal advice. Verify the current text against the Official Gazette. Provisions may have been amended or repealed. Using this page creates no attorney-client relationship. For legal advice, consult a Philippine lawyer.