Depends on the game, but usually around 3-4 GBs. I think most PS3 games offer some sort of installation option.
The trend of mandatory installations is starting to begin though. MGS4 took about ten minutes to install on top of a 1-2 minute install for each new chapter. Even though I have a 60 GB model, I hope that games in the future will only offer it as an option, seeing as how someone with a 20 or 40 GB model could run out of room quickly.
3-4 GBs is not bad considering a blu-ray disc holds 25GBs (single layer) 50GBs (dual layer). Not sure how much room on the disc is taken up by an average game and im not sure which layering the PS3 uses. I'm sure the ps3 uses some type of file compression for the installed games too.
Mandatory installations, hmm. I guess that could be a good and a bad thing.
Xbox Xbox 360 - Broke Xbox 360 Elite =) Playstation 1 Playstation 2 Sega Saturn 2 Sega Dreamcasts - one of them is painted blue and white GameCube Nintendo 64 _ Jungle Green ...I use to have Super NES =/