  We present an observational constraint for the typical active galactic
nucleus (AGN) phase lifetime. The argument is based on the time lag between an
AGN central engine switching on and becoming visible in X-rays, and the time
the AGN then requires to photoionize a large fraction of the host galaxy. Based
on the typical light travel time across massive galaxies, and the observed
fraction of X-ray selected AGN without AGN-photoionized narrow lines, we
estimate that the AGN phase typically lasts $\sim10^{5}$ years. This lifetime
is short compared to the total growth time of $10^{7}-10^{9}$ years estimated
from e.g. the Soltan argument and implies that black holes grow via many such
short bursts and that AGN therefore "flicker" on and off. We discuss some
consequences of this flickering behavior for AGN feedback and the analogy of
X-ray binaries and AGN lifecycles.
