  Non-steady and eruptive phenomena in quasars are thought to be associated
with the Eddington or super-Eddington luminous stage. Although there is no lack
in hypotheses about the total duration of such a stage, the latter remains
essentially unknown. We calculate the duration of quasar luminous phase in
dependence upon the initial mass of a newborn massive black hole (MBH) by
comparing the observed luminosity- and redshift distributions of quasars with
mass distribution of the central MBHs in normal galactic nuclei. It is assumed
that, at the quasar stage, each MBH goes through a single (or recurrent)
phase(s) of accretion with, or close to, the Eddington luminosity. The mass
distributions of quasars is found to be connected with that of MBHs residing in
normal galaxies by a one-to-one corrrespondence through the entire mass range
of the inferred MBHs if the accretion efficiency of mass-to-energy
transformation $\eta \sim 0.1$.
