Some degree of prediction is "always on" I think, therefore it is factored in whenever you are rendering or just re-projecting.
As for how many pixels we are talking about, you can get n idea by looking at the re-projection artifacts in this direct feed video from The London Heist.
https://www.youtube.com/watch?v=m2CXbjwLv2w
Pay very close attention to the top corners when view turns around. Especially the top/left one. It's *very* hard to notice in motion (they seem to be hiding it with some sort of edge mirroring so you don't see black areas popping in) but here are a few frames I've captured that clearly show the artifacts.
Reminds me of the old PS1 games
Anyway... It is worth noting this is a 1+ year old footage. Stuff might have changed since then.
Finally, here is a good video showing re-projection in action on the Rift with VorpX
https://youtu.be/12R8Z4mssoY?t=12m43s