  We present a robust method to derive the duty cycle of QSO activity based on
the empirical QSO luminosity function and on the present-day linear relation
between the masses of supermassive black holes and those of their spheroidal
host stellar systems. It is found that the duty cycle is substantially less
than unity, with characteristic values in the range $3-6\times 10^{-3}$.
Finally, we tested the expectation that the QSO luminosity evolution and the
star formation history should be roughly parallel, as a consequence of the
above--mentioned relation between BH and galaxy masses.
