Next Wednesday, May 27th, Soyuz TMA-15 should launch, marking the beginning of a permanent six person crew (Expedition 20) for the International Space Station, when they dock there a couple of days later. The TMA-15 Soyuz will join the TMA-14 Soyuz which launched March 26th. The six person crew will requires two Soyuz for emergency evacuation, as each only holds three. TMA-14 is planned to return to earth the end of September, after TMA-16 arrives.
As the TMA Soyuz continue to be limited to approximately six months on orbit, we should be seeing a new Soyuz at the station every three months or so, swapping half the crew with each flight.
The TMAT Soyuz, is planned to extend its on orbit duration to a full year, and we’d likely see a reduction in flight frequency if, and when, it enters service.