The Bab said that whenever "He whom God will make manifest" appears, accept Him. He never said don't accept Him until after the lapse of 1000 years. Also Baha'u'llah says that in the year 9 of the Babi Dispensation the time was ripe for the Revelation of "He whom God will make manifest". As the Bab was not only a Manifestation but a Herald of this Baha'i Faith, the interval between His Revelation and that of Baha'u'llah was of shorter duration. His Dispensation in a sense will last as long as Baha'u'llah's lasts.