Sony won't stop the PS2 until the PS3 has a real foothold in the gaming market of today. I can see the PS2 still selling well into 2009, but the games will dry up over the course of this year.
I don't know why people are talking about the PS4, there won't be any signs or proper talk of the PS4 for another 3-4 years anyway as I can see the PS3 leading a very strong and long life of the course.
Timeline
Summer 2008 - Sony announce a final PS2 pricedrop, seeing the UK price dropping to £50-£70 including a game or two.
Winter 2008 - Sees the final few big PS2 game releases WW.
Early 2009 - Sony announces the retiral of the PS2, but states that they will continue to back the system in the meantime.
Late 2009 - PS2 sales are all but dried up, new games are only appearing on the PS3, yet highstreet stores are still offering huge console bundle packs for next to nothing.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)