private function expiration() {

		if ( isset( $this->expiration ) ) {
			return $this->expiration;
		}

		/**
		 * Filters the expiration.
		 *
		 * @since 1.0.0
		 *
		 * @param int $expiration Expiration in seconds.
		 */
		$this->expiration = (int) apply_filters( self::FILTER_EXPIRATION, 300 );

		return $this->expiration;
	}