If you are playing a disc-based game, the system will begin caching the disc when you put it in the console and get ready to play. The game is saving part of itself to the system's hard drive. The amount of data that has to be saved before you can start will vary per title. P
Cerny said that for the launch game he directed, Knack, users should only have to wait 10s of seconds to play the game. After that, as you play, the game will stream more content to the console's 500GB hard drive. Knack will use 37GB of space overall, as noted on the game's box. Obviously, it won't take many games to fill the console's hard drive.P
Cached/installed game data will stay on the hard drive until the user deletes it. Cerny said that there had been some internal discussions at Sony about having the PS4 auto-delete installed data from games that players hadn't used in a while. They decided against it, figuring that gamers would never want to feel "blindsided" and would prefer to make their data management decisions manually. Probably a good choice!