  Accretion disc theory predicts that an AGN disc becomes self-gravitating and
breaks up into stars at an outer radius $R_{\rm sg}$ ~ 12 light-days, with
effectively no free parameter. We present evidence that the longer observed AGN
light echoes are all close to 12d in the AGN rest frames. These observations
give a stringent test of AGN disc theory. Further monitoring should offer
insight into the formation angular momentum of the gas forming the disc. For
distant AGN, observed lags significantly longer than 12d give lower limits on
their redshifts.
