What is the highest plausible redshift of luminous quasars?

The recent discoveries of luminous quasars at high redshifts imply that bla ck holes more massive than a few billion solar masses were already assemble d when the universe was less than a billion years old. We show that the exi stence of these black holes is not surprising in popular hierarchical model s of structure formation. For example, the black hole needed to power the q uasar SDSS 1044-0125 at z = 5.8 could arise naturally from the growth of st ellar-mass seeds forming at z > 10, when typical values are assumed for the radiative accretion efficiency (similar to0.1) and the bolometric accretio n luminosity in Eddington units (similar to1). Nevertheless, SDSS 1044-0125 yields a nontrivial constraint on a combination of these parameters. Extra polating our model to future surveys, we derive the highest plausible redsh ift for quasars that are not lensed or beamed, as a function of their appar ent magnitude. We find that at a limiting magnitude of K similar to 20, qua sar surveys can yield strong constraints on the growth of supermassive blac k holes out to z similar to 10.