The existence of luminous quasars hosting supermassive black holes within the first billion years of cosmic history challenges our understanding of black hole growth. An important piece to the puzzle is the lifetime of quasars, the time that black holes shine as active quasars and during which the bulk of the black hole growth occurs. However, theoretical models of supermassive black hole growth are challenged by the multi-scale physics involved, while most observational constraints on the quasar lifetime cannot be applied beyond redshifts of z>4 due to the low number density of high-redshift quasars. I will present a novel method to measure the lifetime of high-redshift quasars based on the extent of the ionized regions around them, which can act as a "quasar clock" and thus enable us to study the timescales of black hole accretion at early cosmic times. Surprisingly, we find that black holes can grow on much shorter timescales than expected, providing a potential solution to the long-standing puzzle of early black hole growth. I will show how we aim to understand the dominant processes that govern black hole growth using a combination of multi-wavelength observations and data-driven models. Finally, I will present some of the first results from the EIGER collaboration using JWST/NIRCam observations in wide-field stitless grism mode of high-redshift quasars.