No way it takes them as long into life cycle to release as gt5 took. It will look more like when 3 was released for ps2. So my bet is on q2 2015.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut