  The science objectives of the LISA mission have been defined under the
implicit assumption of a 4 yr continuous data stream. Based on the performance
of LISA Pathfinder, it is now expected that LISA will have a duty cycle of
$\approx 0.75$, which would reduce the effective span of usable data to 3 yr.
This paper reports the results of a study by the LISA Science Group, which was
charged with assessing the additional science return of increasing the mission
lifetime. We explore various observational scenarios to assess the impact of
mission duration on the main science objectives of the mission. We find that
the science investigations most affected by mission duration concern the search
for seed black holes at cosmic dawn, as well as the study of stellar-origin
black holes and of their formation channels via multi-band and multi-messenger
observations. We conclude that an extension to 6 yr of mission operations is
recommended.
