Unless ND tool up with more personnel, then they will probably continue the 2 year game cycle for the foreseeable future.
Seeing as we will see the 2nd Uncharted within the PS3's 3rd year of a long cycle, I see no reason (unless U2 bombs badly) for a 3rd to appear on the PS3 within the next 3/4 years.
I have the sneaky feeling though that ND have been slowly developing a Jak & Daxter game for a little while now and may have it ready for release late next year.
Although through all this, I also get the feeling that they may try and create a new IP, and I reckon Sony will back them on this looking at the success of Uncharted and what U2 looks like it may do.
So my prediction is this:
October 2009 - Release Uncharted 2
Late 2009 to early 2010 - Release patches and multiplayer levels for Uncharted 2
Late 2010 - Release Jak & Daxter
Late 2011 - Release new IP
Late 2012 - Release Uncharted 3
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)