A February 17, 2014 story in NASASpaceFlight.com reports that NASA’s Associate Administrator for Human Exploration and Operations Bill Gerstenmaier has stated that the space agency’s heavy lift Space Launch System must fly at least once a year in order to maintain “repetitive cadence,” which involves keeping launch crews in practice. Thus far only two launches are planned, one an unmanned test flight in 2017 and a visit to a captured asteroid in 2021. It is noted that as currently configured, the SLS can launch twice a year, given funding and available payloads.
There is no end to ideas of missions that could use the SLS, ranging from lunar surface expeditions to heavy, robotic probes to the Outer Planets. Eventually a version of the SLS will be used to send people to Mars. NASA is looking to foreign countries to fly missions on the SLS to help bump up the flight rate.
The dirty not so secret about the SLS is that more funding, from some source, is needed to make full use of the capabilities it will impart. There is little to no prospect of that happening during the rest of the Obama administration. It is possible, though, that a new president will be moved to give the matter a fresh look.