  The variability of quasar light curves can be used to study the structure of
quasar accretion disks. For example, continuum reverberation mapping uses
delays between variability in short and long wavelength bands ("short" lags) to
measure the radial extent and temperature profile of the disk. Recently, a
potential reverse lag, where variations in shorter wavelength bands lag the
longer wavelength bands at the much longer viscous timescale, was detected for
Fairall 9. Inspired by this detection, we derive a timescale for these "long"
negative lags from fluctuation propagation models and recent simulations. We
use this timescale to forecast our ability to detect long lags using the Vera
Rubin Legacy Survey of Space and Time (LSST). After exploring several methods,
including the interpolated cross-correlation function, a Von-Neumann estimator,
javelin, and a maximum-likelihood Fourier method, we find that our two main
methods, javelin and the maximum-likelihood method, can together detect long
lags of up to several hundred days in mock LSST light curves. Our methods work
best on proposed LSST cadences with long season lengths, but can also work for
the current baseline LSST cadence, especially if we add observations from other
optical telescopes during seasonal gaps. We find that LSST has the potential to
detect dozens to hundreds of additional long lags. Detecting these long lags
can teach us about the vertical structure of quasar disks and how it scales
with different quasar properties.
