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.