Baha'u'llah reveals himself only once every 500,000 years

“Erelong you will hear the call of the clamorous impostor, don’t pay attention to him, but leave him to himself. Set your faces toward Him Who is the Object (the Qiblih) of the adoration of all mankind, who has fulfilled the proof with this Proof that is revealed through the power of truth (God, al-Haqq). The Lights have attained their consummation in this source (horizon) whence the splendor of (the) Might and (the) Grandeur is diffused. Blessed is the soul that hath been brought up as the servant with respect to God in that which has been sent down in the Scrolls and the Tablets. Say: If in each day another one is manifested, the cause of God will not find rest in the cities and the lands. This is the Revelation (Manifestation) that manifests itself (His own Self) every 500,000 years time only once. Moreover, we unveiled the cover and we lifted the veil. Blessed is the man that recognized (‘arif) the purpose of God. In his recognition (‘irafah) has rejoiced in his heart and been made firm in the cause such that he cannot be made to stumble by the entire creation. Verily We unveiled in this Tablet a secret wrapt in secrets. This Revelation We have concealed that which is the Hidden lest the tumult of the wicked rise.”

http://www.hajir.org/?p=37