A revoke becomes established (i.e. it cannot be corrected) when one of the offending side leads or plays to the next trick. A revoke on the twelfth trick never becomes established.
Penalties apply to revokes and one should try hard to avoid making them. If partner shows out of a suit, you may enquire ‘having none?’ to check that the play is not a revoke. This applies whether you are a defender or dummy.