  We expect luminous ($M_{1450}\lesssim-26.5$) high-redshift quasars to trace
the highest density peaks in our universe, and therefore to reside in
proto-clusters encompassing an abundance of galaxies in close vicinity. Here,
we present observations of four $z\gtrsim6$ quasar fields using JWST/NIRCam in
imaging and widefield slitless spectroscopy mode and report a wide range in the
number of detected [OIII]-emitting galaxies in the quasars' environments,
ranging between a density enhancement of $\delta>100$ within a $2$ cMpc radius
- one of the largest proto-clusters during the Epoch of Reionization discovered
to date - to a density contrast consistent with zero, indicating the presence
of a UV-luminous quasar in a region comparable to the average density of the
universe. By measuring the two-point cross-correlation function of quasars and
their surrounding galaxies, as well as the galaxy auto-correlation function, we
infer a correlation length of quasars at $\langle z\rangle=6.25$ of $r_0^{\rm
QQ}=21.3^{+2.7}_{-2.6}~{\rm cMpc}\,h^{-1}$, while we obtain a correlation
length of the [OIII]-emitting galaxies of $r_0^{\rm GG}=4.2\pm0.1~{\rm
cMpc}\,h^{-1}$. By comparing the correlation functions to dark-matter-only
simulations we estimate the minimum mass of the quasars' host dark matter halos
to be $\log_{10}(M_{\rm halo, min}/M_\odot)=12.30\pm0.14$ (and
$\log_{10}(M_{\rm halo, min}^{\rm [OIII]}/M_\odot) = 10.72\pm0.03$ for the
[OIII]-emitters), indicating that (a) luminous quasars do not necessarily
reside within the most overdense regions in the early universe, and that (b)
the UV-luminous duty cycle of quasar activity at these redshifts is $f_{\rm
duty}\ll1$. Such short quasar activity timescales challenge our understanding
of early supermassive black hole growth and provide evidence for highly
dust-obscured growth phases or episodic, radiatively inefficient accretion
rates.
