"How Long do Quasars Shine?"

Luminous quasars are believed to be the progenitors of the supermassive black holes observed ubiquitously at the centers of all massive galaxies, but we are still in the dark about how these black holes formed. Our ignorance largely results from the fact that the expected timescale for supermassive black hole growth of 50 million years is far longer than the mere fifty years that humans have been observing quasars. A holy grail would thus be a direct measurement of quasar lifetimes, shedding light on the physical mechanisms responsible for fueling black hole growth, and how the back-reaction of this growth might influence how galaxies form. I will discuss two very different experiments that allow us to construct cosmic clocks that can accurately time the duration of luminous quasar activity on timescales of kiloyears to gigayears. One exploits the clustering pattern of quasars on the sky, which has recently been measured by JWST. The other uses observations of diffuse intergalactic gas in quasar environs. I will also touch upon how the latter can be used to constrain the reionization history of the Universe.