Just because Scott Kelly has landed and retired doesn't mean that NASA is done with long-term stays in space. The agency informs Ars Technica that it's aiming for five more year-long missions aboard the International Space Station, with the first starting as early as September 2018 and the last finishing by the station's expected shutdown in 2024. There may be some overlap to fit all of them into the schedule.
Why not longer, to gauge how astronauts would fare on trips to Mars? It's not practical, NASA says. There would likely be just one 2- or 3-year mission in the time the ISS has left, and that mission would play havoc with the remaining crew rotations. Even if NASA could stuff two missions into that period, that wouldn't exactly provide a lot of data. The single-year intervals strike a balance between longevity and the volume of research. As it is, ISS visits can't illustrate the effects of communication delays and radiation by themselves.
The big question is whether or not NASA will get the funding it needs to complete such a mission. President-elect Trump senior advisor Bob Walker tells the Guardian that NASA under Trump will focus on on an "exploration role," which could include these ISS missions. However, there's no guarantee that the administration will see station visits this way. It might scale operations back if it thinks ISS trips contribute to the study of human-made climate change. We're not expecting a sudden change of heart, but there's going to be a lot of uncertainty until the first Trump-era NASA budget arrives.