I can't speak for all games... but I know most are around the 2-4GB per game variety. Borderlands (with all the expansions) is around 6GB of an install, while still requiring the disk.
Gran Turismo I don't have, but I understand it's rather infamous with the amount of surreptitious installs of data onto the hard drive. Frankly, one of the biggest arguments for an SSD was to shorten those install and load times for THIS GAME!
I just know on average, when I am able to try a game that is on both systems, the mandatory/optional installs for the PS3 only seem to bring it to have the same loading times as it would be on 360 if I just played from its disc. To my understanding, only 6 games in the entire 360 catalog are defaulted to not support the NXE install feature, because they were the few that actually turned out to perform worse this way. IIRC, none of those were multiplatform games, though.--
Because, f*ck Sony