Before the loss of Columbia during the STS-107 mission on February 1 2003, the International Space Station was scheduled to reach "Core Complete" status in February 2004. (Although this was increasingly unlikely at the time, and the Columbia Accident Investigation Board cited this schedule pressure as a contributing factor to the accident). The ISS Assembly Complete planned date at that time was likely a few years further on, perhaps 2005 (also increasingly unlikely).
That being said, were any shuttle manifests or plans ever published showing proposed utilization of the fleet after ISS Assembly Complete? Would the schedule have been reduced to two or three launches a year to rotate ISS crewmembers and refresh supplies? Or would the Orbiters have, in addition to ISS support, returned to launching other payloads?
I don't ever remember seeing this addressed, perhaps because before STS-107 the focus was solely on Core and Assembly Complete, and after STS-107 everything was thrown into Return to Flight and then Assembly complete. But surely NASA did some long range planning?