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

Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ~1e5 years