We analyzed spectra of 15 luminous high-redshift quasars (3.3<z<5.1) and we derived black hole masses that are based on Hβ, MgII, and CIV emission lines, in the range of $2\times10^8<M_{bh}<4\times10^{10} M_\odot$. Assuming exponential growth of the black holes we estimated that it takes about several 100 Myr up to 1 Gyr to accumulate the observed black hole masses, i.e., most of the SMBHs that power these quasars at high-redshifts, started to grow at $z \simeq 5 {\rm to} 10$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html